Class 10 Science Unit 9: Light – Reflection and Refraction
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Light is one of the most fascinating topics in science because it helps us see the world around us and also behaves in surprising ways. It travels extremely fast, reflects from surfaces, bends when it passes from one medium to another, and forms images that can be real or virtual. The chapter Light – Reflection and Refraction is one of the most important chapters in Class 10 Science because it introduces the basic rules of geometrical optics. These ideas are used in mirrors, lenses, telescopes, microscopes, spectacles, cameras, projectors, and many other devices.
This chapter explains how light behaves when it falls on smooth surfaces and when it passes through transparent substances. Reflection tells us how light bounces back from mirrors. Refraction explains how light bends as it moves from air to glass or from glass to water. By studying this chapter carefully, students learn how to construct ray diagrams, use mirror and lens formulas, determine image positions, and understand optical instruments. It is a highly scoring chapter because many of its questions are formula-based, diagram-based, and concept-based.
The chapter is not just about memorizing rules. It is about understanding how light interacts with matter. The laws of reflection and refraction are beautifully logical, and once the underlying geometry is understood, many problems become easy to solve. The chapter also develops scientific thinking because students must learn to observe, draw, and calculate. This makes it one of the most useful and elegant chapters in the science curriculum.
What Is Light?
Light is a form of energy that enables us to see objects. It travels in straight lines in a homogeneous medium and shows wave-like as well as particle-like behaviour in physics. In this chapter, however, we study light mainly as a ray that follows geometric rules. This part of optics is called geometrical optics or ray optics.
Light is emitted by luminous objects such as the Sun, bulbs, flames, and LEDs. Non-luminous objects do not produce light themselves but become visible when they reflect light from a luminous source. The human eye detects light and forms visual perception. Without light, seeing would not be possible.
Light behaves in special ways when it strikes a mirror or passes into a different medium. These behaviours help us understand mirrors, lenses, and image formation. The chapter mainly deals with reflection from mirrors and refraction through transparent materials.
Reflection of Light
Reflection is the bouncing back of light into the same medium after striking a surface. When light falls on a mirror, it does not pass through it; instead, it comes back after changing direction. This is why mirrors can form images.
Reflection occurs from many surfaces, but smooth and polished surfaces produce regular reflection, while rough surfaces produce diffuse reflection. In a plane mirror, reflection is regular and image formation is clear. In a rough wall, reflected rays scatter in different directions, so no clear image is formed.
Laws of Reflection
Reflection follows two basic laws. These laws are always true for a reflecting surface.
- The angle of incidence is equal to the angle of reflection.
- The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
The angle of incidence is the angle between the incoming ray and the normal. The angle of reflection is the angle between the reflected ray and the normal. The normal is a line drawn perpendicular to the surface at the point where light strikes.
These laws explain how mirrors work and help in drawing ray diagrams. They are fundamental rules in optics and are used repeatedly in this chapter.
Plane Mirror
A plane mirror is a flat, smooth mirror. It forms a virtual image that is upright, laterally inverted, and of the same size as the object. The image appears behind the mirror at the same distance as the object is in front of it. This property makes plane mirrors simple yet very useful.
Characteristics of Image Formed by a Plane Mirror
- The image is virtual and cannot be obtained on a screen.
- The image is erect.
- The image is of the same size as the object.
- The image is formed behind the mirror.
- The image is laterally inverted.
- The image is as far behind the mirror as the object is in front of it.
Lateral inversion means the left side of the object appears as the right side in the image and vice versa. This is why writing looks reversed in a mirror. Plane mirrors are used in dressing mirrors, periscopes, and many optical devices.
Spherical Mirrors
Spherical mirrors are mirrors whose reflecting surfaces form part of a sphere. They are of two types: concave mirrors and convex mirrors. A concave mirror curves inward like a spoon, while a convex mirror curves outward.
Concave Mirror
A concave mirror is a spherical mirror in which the reflecting surface is on the inner side of the spherical surface. It can form both real and virtual images depending on the position of the object.
Concave mirrors converge parallel rays of light to a point called the focus. Therefore, they are also called converging mirrors. They are used in headlights, torches, shaving mirrors, solar furnaces, and reflectors.
Convex Mirror
A convex mirror has its reflecting surface on the outer side of the spherical surface. It diverges parallel rays of light and always forms a virtual, erect, and diminished image. Convex mirrors are used in rear-view mirrors and security mirrors because they provide a wide field of view.
Important Terms in Spherical Mirrors
To study spherical mirrors properly, several important points and terms are needed.
- Pole: The midpoint of the reflecting surface of the mirror.
- Centre of curvature: The centre of the sphere of which the mirror is a part.
- Radius of curvature: The distance between the pole and the centre of curvature.
- Principal axis: The straight line passing through the pole and the centre of curvature.
- Principal focus: The point where rays parallel to the principal axis meet after reflection in a concave mirror, or appear to diverge from in a convex mirror.
- Focal length: The distance between the pole and the principal focus.
For spherical mirrors, the focal length is equal to half the radius of curvature. This relation is important in calculations and ray diagrams.
Ray Diagrams for Spherical Mirrors
Ray diagrams help us understand where an image is formed and what its characteristics are. To draw them, we use a few basic rules about rays of light. These rules apply to both concave and convex mirrors.
Common Rays for Mirror Diagrams
- A ray parallel to the principal axis passes through the focus after reflection in a concave mirror.
- A ray passing through the focus reflects parallel to the principal axis.
- A ray passing through the centre of curvature reflects back along the same path.
- A ray striking the pole obeys the law of reflection.
These rules make it possible to locate images for different object positions. A concave mirror can produce different types of images depending on where the object is placed. A convex mirror, on the other hand, always forms a virtual, erect, diminished image.
Image Formation by a Concave Mirror
Concave mirrors are especially interesting because they can form different kinds of images. The image may be real or virtual, inverted or erect, magnified or diminished. This depends on the object position relative to the focus and centre of curvature.
- If the object is very far away, the image is formed at the focus and is real, inverted, and very small.
- If the object is beyond the centre of curvature, the image forms between the centre of curvature and the focus, and it is real, inverted, and diminished.
- If the object is at the centre of curvature, the image forms at the centre of curvature, is real, inverted, and same size.
- If the object is between the centre of curvature and the focus, the image forms beyond the centre of curvature and is real, inverted, and magnified.
- If the object is at the focus, the image is formed at infinity and is highly enlarged.
- If the object is between the focus and the pole, the image is virtual, erect, and enlarged behind the mirror.
These cases are frequently asked in exams. The behaviour of the concave mirror is the basis of many useful devices.
Uses of Concave Mirrors
Concave mirrors are used when we need a magnified image or a concentrated beam of light. They are used in shaving mirrors, dentist mirrors, headlights, torches, searchlights, and solar cookers. Their converging property makes them highly practical.
When an object is placed near a concave mirror, a magnified upright image is formed. This is useful for shaving or applying makeup. In headlights and torches, a light source placed at the focus produces a nearly parallel beam of light.
Image Formation by a Convex Mirror
Convex mirrors always form a virtual, erect, and diminished image regardless of the position of the object. The image appears behind the mirror between the pole and the focus. Because the image is diminished, the mirror provides a wider field of view.
This property makes convex mirrors very useful in vehicles as rear-view mirrors. Drivers can see a larger area of the road behind them. Convex mirrors are also used in shops, parking areas, and security systems.
Sign Convention for Mirrors
To solve numerical problems involving mirrors, a standard sign convention is used. In this convention, the pole is taken as the origin, and distances measured in the direction of incident light are positive, while those measured opposite to it are negative. Heights measured above the principal axis are positive, and heights below it are negative.
This convention helps avoid confusion in calculations. Students must use it carefully when applying mirror formulas.
Mirror Formula
The mirror formula relates the object distance, image distance, and focal length of a spherical mirror. It is written as:
1/f = 1/v + 1/u
Here, f is the focal length, v is the image distance, and u is the object distance. This formula is very useful in finding the position of images formed by concave and convex mirrors.
Students should use the correct sign convention while applying this formula. If the signs are handled properly, the result becomes easy to interpret.
Magnification by Mirrors
Magnification tells us how much larger or smaller the image is compared to the object. It is given by:
m = h' / h = -v / u
Here, h' is the height of the image and h is the height of the object. For mirrors, a negative magnification means the image is inverted, while a positive value means the image is erect.
Magnification helps describe image size mathematically and is often used in numerical questions.
Refraction of Light
Refraction is the bending of light when it passes from one medium to another. This happens because the speed of light changes in different media. For example, light travels at different speeds in air, water, glass, and other transparent materials.
When light enters a denser medium from a rarer medium, it bends towards the normal. When it moves from a denser medium to a rarer medium, it bends away from the normal. Refraction is the reason why a pencil looks bent in water, why a coin appears raised in a bowl, and why lenses can form images.
Laws of Refraction
Refraction follows two laws, also called Snell’s laws.
- The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.
- For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.
This constant is called the refractive index. It tells us how much light slows down and bends in a particular medium.
Refractive Index
Refractive index is the measure of how much a medium bends light. It is defined as the ratio of the speed of light in one medium to the speed of light in another. A higher refractive index means light travels slower in that medium and bends more strongly.
Refractive index helps compare materials such as air, water, glass, and diamond. Diamond has a high refractive index and bends light strongly, which is one reason why it sparkles.
Refraction Through a Rectangular Glass Slab
When light passes through a rectangular glass slab, it bends towards the normal on entering the glass and away from the normal on leaving it. The emergent ray is parallel to the incident ray but shifted sideways. This sideways shift is called lateral displacement.
The rectangular glass slab experiment shows that light changes direction twice, but the final direction remains parallel to the original direction. This is a standard practical demonstration of refraction.
Lenses
Lenses are transparent materials, usually glass or plastic, that refract light and are used to form images. They are used in spectacles, cameras, microscopes, magnifying glasses, projectors, and telescopes. Lenses are of two main types: convex lenses and concave lenses.
Convex Lens
A convex lens is thicker at the centre and thinner at the edges. It converges parallel rays of light to a focus. Therefore, it is called a converging lens. It can form real or virtual images depending on the object position.
Concave Lens
A concave lens is thinner at the centre and thicker at the edges. It diverges parallel rays of light. Therefore, it is called a diverging lens. It always forms a virtual, erect, and diminished image.
Important Terms Related to Lenses
- Optical centre: The central point of a lens through which a ray passes undeviated under certain conditions.
- Principal axis: The line joining the centres of curvature of the lens surfaces.
- Principal focus: The point where rays parallel to the principal axis converge or appear to diverge from after refraction.
- Focal length: The distance between the optical centre and the principal focus.
Image Formation by a Convex Lens
A convex lens can form different types of images depending on the position of the object. If the object is very far away, the image forms at the focus and is real, inverted, and highly diminished. If the object is beyond the focus, the image can be real and inverted. If the object is between the focus and the lens, the image is virtual, erect, and magnified.
Convex lenses are used in magnifying glasses and optical instruments because of their ability to produce magnified images.
Image Formation by a Concave Lens
A concave lens always forms a virtual, erect, and diminished image. The image appears on the same side as the object and between the lens and its focus. This lens is often used in spectacles to correct short-sightedness.
Lens Formula
The lens formula is:
1/f = 1/v - 1/u
Here, f is the focal length, v is the image distance, and u is the object distance. Like the mirror formula, this formula also uses a sign convention. It is essential for solving lens-based numerical problems.
Magnification by Lenses
Magnification by a lens is given by:
m = h' / h = v / u
The sign and magnitude of magnification tell us whether the image is erect or inverted and whether it is enlarged or diminished.
Power of a Lens
The power of a lens tells us how strongly it converges or diverges light. It is defined as the reciprocal of focal length in metres.
P = 1/f
The unit of power is dioptre. A lens with short focal length has greater power. Convex lenses have positive power, and concave lenses have negative power.
Uses of Lenses
Convex lenses are used in cameras, projectors, microscopes, telescopes, and magnifying glasses. Concave lenses are used in spectacles for short-sightedness, door viewers, and some optical instruments. The ability of lenses to bend light makes them very important in modern technology.
Practical Concepts and Experimental Ideas
Several ideas in this chapter are connected with experiments and observation. For example, the law of reflection can be demonstrated using a plane mirror and rays of light. Refraction can be observed by placing a pencil in water or by passing light through a glass slab. Image formation can be studied using concave mirrors and lenses. These observations make the chapter practical and easy to understand.
Students should understand how to draw ray diagrams carefully. A ray diagram is not only a picture; it is a scientific way of predicting where an image will form and what its nature will be. The more practice students do, the easier it becomes to answer numerical and diagram-based questions.
Important Differences to Remember
Students are often asked to compare terms in this chapter. The main differences include:
- Reflection vs Refraction: Reflection is the bouncing back of light; refraction is bending of light in a new medium.
- Concave mirror vs Convex mirror: Concave mirrors converge light and can form real or virtual images; convex mirrors diverge light and form only virtual images.
- Convex lens vs Concave lens: Convex lenses converge light; concave lenses diverge light.
- Real image vs Virtual image: Real images can be obtained on a screen; virtual images cannot.
- Mirror vs Lens: A mirror reflects light, while a lens refracts light.
Important Terms to Remember
- Light: A form of energy that enables vision.
- Reflection: Bouncing back of light from a surface.
- Refraction: Bending of light when it passes from one medium to another.
- Normal: A perpendicular line drawn at the point of incidence.
- Incident ray: The ray of light falling on a surface.
- Reflected ray: The ray that bounces back after reflection.
- Refracted ray: The ray that changes direction while passing through a medium.
- Concave mirror: A mirror with inward-curved reflecting surface.
- Convex mirror: A mirror with outward-curved reflecting surface.
- Concave lens: A lens thinner at the centre and thicker at the edges.
- Convex lens: A lens thicker at the centre and thinner at the edges.
- Focal length: Distance between pole/optical centre and focus.
- Magnification: Ratio of image size to object size.
- Power of lens: Measure of the lens’s ability to converge or diverge light.
Class 10 Science Unit 9 Notes PDF
📄 Download PDFExam-Oriented Revision Points
Students should remember the laws of reflection and refraction, image characteristics of plane mirrors, concave mirrors, convex mirrors, convex lenses, and concave lenses, the mirror and lens formulas, magnification, and power of lenses. Diagrams are extremely important in this chapter. Ray diagrams for different object positions must be practiced carefully.
In numerical questions, use the correct sign convention and formula. In theory questions, write clear definitions and explain the image nature with terms such as real, virtual, erect, inverted, enlarged, and diminished. It is also useful to remember practical applications such as rear-view mirrors, shaving mirrors, spectacles, and magnifying glasses.
This chapter is highly useful because it explains how light works in the tools and devices we use every day. With steady practice, students can master the diagrams and formulas and score very well in exams.
Conclusion
Light – Reflection and Refraction is one of the most important and elegant chapters in Class 10 Science. It explains how light reflects from mirrors and bends in different media, how images are formed, and how optical instruments work. The chapter combines scientific laws with practical observation, making it both logical and visually interesting. By understanding reflection, refraction, spherical mirrors, lenses, and the related formulas, students gain a solid foundation in optics.
The chapter is also important because it has many applications in daily life, from mirrors and glasses to cameras and telescopes. It helps students understand not only how we see objects but also how scientific instruments use light to extend human vision. A clear understanding of this chapter strengthens problem-solving skills, builds diagram-drawing confidence, and prepares students well for board examinations and future science studies.
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