Light Mirrors and Lenses
We see objects because light bounces off them into our eyes. This Class 8 Curiosity chapter explains how light reflects, the laws that govern reflection, and how curved mirrors and lenses bend or gather light to form images — some upright and some inverted, some real and some virtual.
Learning objectives
- State the laws of reflection of light.
- Distinguish regular from diffuse reflection.
- Describe images formed by plane and spherical mirrors.
- Compare how convex and concave lenses act on light.
Key concepts
Reflection and its laws
When light strikes a smooth surface it bounces back; this is reflection. The incident ray, the reflected ray and the normal (the perpendicular at the point of incidence) all lie in one plane, and the angle of incidence always equals the angle of reflection.
Regular and diffuse reflection
A smooth, polished surface like a mirror reflects parallel rays in one direction, giving regular reflection and a clear image. A rough surface scatters rays in many directions, giving diffuse reflection and no clear image — which is why we cannot see ourselves in a wall.
Plane and spherical mirrors
A plane mirror forms an image that is erect, the same size, virtual, and laterally inverted (left and right swapped), as far behind the mirror as the object is in front. A concave (converging) mirror can form enlarged or real images depending on distance, while a convex (diverging) mirror always forms a small, erect, virtual image and gives a wide view.
Lenses
A lens refracts (bends) light passing through it. A convex lens is thicker in the middle and converges light, able to form real or magnified images, while a concave lens is thinner in the middle and diverges light, always forming a small, erect, virtual image. Lenses are used in spectacles, magnifiers and cameras.
Key definitions
- Reflection
- The bouncing back of light when it strikes a surface.
- Normal
- The line drawn perpendicular to a surface at the point where light strikes it.
- Lateral inversion
- The left–right interchange of an image seen in a plane mirror.
- Virtual image
- An image that cannot be caught on a screen, formed where rays only appear to meet.
Solved examples
Q1. The angle of incidence is 35°. What is the angle of reflection?
Solution: By the law of reflection, the angle of reflection equals the angle of incidence, so it is 35°.
Q2. Why can you read text in a mirror only when it is reversed?
Solution: A plane mirror laterally inverts the image, swapping left and right, so normal text appears reversed.
Q3. Which mirror is used as a vehicle rear-view mirror, and why?
Solution: A convex mirror, because it always gives an erect, diminished image and a wider field of view.
Common mistakes to avoid
- Measuring angles of incidence and reflection from the surface instead of from the normal.
- Calling a plane-mirror image 'real' — it is virtual and cannot be caught on a screen.
- Thinking a convex lens always magnifies — it can also form small, inverted real images.
- Confusing reflection (mirrors, bouncing) with refraction (lenses, bending through).
Light Mirrors and Lenses — MCQ Quiz
10 questions with instant feedback. Use number keys 1–4 to answer.
Reflection means light:
Practice questions
Short answer
State the two laws of reflection.
The incident ray, reflected ray and normal lie in one plane; and the angle of incidence equals the angle of reflection.
Give one difference between regular and diffuse reflection.
Regular reflection from a smooth surface gives a clear image; diffuse reflection from a rough surface scatters light and gives none.
Why is a convex mirror used in vehicles?
It always forms an erect, diminished image and covers a wider area behind the vehicle.
Long answer
Describe the image formed by a plane mirror and list its characteristics.
A plane mirror forms an image that is virtual (cannot be caught on a screen), erect, and the same size as the object. It is laterally inverted, with left and right interchanged, and is located as far behind the mirror as the object is in front of it.
Compare convex and concave lenses in how they treat light and the images they form.
A convex lens is thicker in the middle and converges parallel light to a point; depending on the object's distance it can form a real, inverted image or a magnified, erect virtual image (as in a magnifying glass). A concave lens is thinner in the middle and diverges light, always forming a small, erect, virtual image. Convex lenses are used in cameras and magnifiers; concave lenses help correct short-sightedness.
HOTS (Higher Order Thinking)
When you raise your right hand before a plane mirror, the image raises its 'left'. Explain.
This is lateral inversion: the plane mirror swaps left and right, so your right hand appears as the image's left hand.
Why does a polished steel plate show your face but a sheet of paper does not?
The smooth steel gives regular reflection, sending rays in one direction to form an image; the rough paper scatters light by diffuse reflection, so no clear image forms.
Quick revision
Revision notes
- Laws of reflection: rays and normal in one plane; angle i = angle r.
- Smooth surface → regular reflection; rough → diffuse.
- Plane mirror: virtual, erect, same size, laterally inverted.
- Convex lens converges (real/magnified images); concave lens diverges (small, erect, virtual).
Key takeaways
- Angles are always measured from the normal.
- Mirrors reflect light; lenses refract it.
- Convex converges, concave diverges — for both mirrors and lenses.
Frequently asked questions
What are the laws of reflection?
The incident ray, reflected ray and normal lie in the same plane, and the angle of incidence equals the angle of reflection.
Is the image in a plane mirror real or virtual?
Virtual — it appears behind the mirror and cannot be projected onto a screen.
What is the difference between a convex and a concave lens?
A convex lens is thicker in the middle and converges light; a concave lens is thinner in the middle and diverges it.