Light — Class 8

Why We Study Light

Light is what lets us see the world. But it’s also a physical thing — it travels in straight lines, bounces off mirrors, and behaves predictably enough that we can build microscopes, telescopes, and cameras around it. Class 8 introduces the basic rules of light, which become the foundation for ray optics in Classes 10 and 12.

The big questions we’ll answer: Why do mirrors form images? Why is the world upside-down inside our eyes (and how does our brain fix it)? Why are some animals colour-blind? And how does the dispersion of light through a prism create a rainbow?

Key Terms & Definitions

Light: A form of energy that travels in straight lines (called rays) and lets us see objects.

Reflection: The bouncing back of light from a surface.

Incident ray: The ray of light hitting a surface.

Reflected ray: The ray of light bouncing off the surface.

Normal: A line perpendicular (90°) to the surface at the point where the ray hits.

Angle of incidence: Angle between the incident ray and the normal.

Angle of reflection: Angle between the reflected ray and the normal.

Regular reflection: Reflection from a smooth surface (like a mirror) — parallel rays remain parallel.

Diffuse reflection: Reflection from a rough surface — parallel rays scatter in different directions.

Laws of Reflection

The angle of incidence equals the angle of reflection: $\angle i = \angle r$ .
The incident ray, the normal, and the reflected ray all lie in the same plane.

These two laws govern every reflection — from a mirror, from water, from a polished spoon.

Mirrors

Plane Mirror (Flat Mirror)

The image formed by a plane mirror is:

The same size as the object
As far behind the mirror as the object is in front
Laterally inverted (left becomes right and vice versa)
Virtual (cannot be caught on a screen)
Erect (right-side-up)

This is why text written on your shirt appears mirror-reversed in a bathroom mirror.

Spherical Mirrors

Curved mirrors come in two types:

Concave mirror: curved inward (like the inside of a spoon). Used in shaving mirrors, telescopes, headlight reflectors.
Convex mirror: curved outward (like the back of a spoon). Used in vehicle rear-view mirrors, store security mirrors.

Concave mirrors can form real images (when the object is far enough). Convex mirrors always form virtual, diminished images — useful because they show a wider field of view.

Refraction (Brief)

Light bends when it passes from one medium to another (e.g., from air into water). This is why a pencil dipped in water looks “broken” at the surface.

The amount of bending depends on the change in speed of light. We don’t need formulas at Class 8 — just the observation that light bends when it changes medium.

Dispersion of Light

When white light passes through a prism, it splits into seven colours: violet, indigo, blue, green, yellow, orange, red (mnemonic: VIBGYOR).

This happens because different colours of light bend by different amounts in glass. Violet bends most, red bends least. So they exit at different angles and we see a spectrum.

A rainbow is nature’s prism: water droplets split sunlight into colours.

The Human Eye

The human eye is a camera-like organ:

Cornea: transparent front cover; does most of the focusing.
Iris: coloured part; controls the size of the pupil.
Pupil: opening that lets light in (gets smaller in bright light).
Lens: fine-tunes focus by changing shape (called accommodation).
Retina: light-sensitive layer at the back; contains rods and cones.
Optic nerve: carries signals to the brain.

Rods: detect dim light, no colour information.

Cones: detect bright light and colour. Three types — sensitive to red, green, and blue.

The image formed on the retina is upside-down. Our brain flips it right-side up before we become aware of it.

Persistence of Vision

The eye retains an image for about $\frac{1}{16}$ of a second after the light source is removed. This is why movies (24 frames per second) appear as continuous motion.

Defects of Vision

Myopia (short-sightedness): distant objects appear blurry. Corrected with concave (diverging) lenses.
Hypermetropia (long-sightedness): nearby objects appear blurry. Corrected with convex (converging) lenses.
Cataract: clouding of the lens. Treated by surgical replacement.

Solved Examples

Easy

A ray of light strikes a plane mirror at 30° from the surface. What is the angle of reflection?

The angle of incidence is measured from the normal, not the surface. So if the ray is 30° from the surface, it’s 60° from the normal. By the law of reflection, the angle of reflection is also 60° (from the normal).

Easy

You are 1.5 m in front of a plane mirror. How far is your image from you?

Image is 1.5 m behind the mirror. Distance from you to image = 1.5 + 1.5 = 3.0 m.

Medium

Why does a convex mirror give a wider field of view than a plane mirror?

Convex mirrors curve outward, so they reflect rays from a wider region back to your eye. Plane mirrors only reflect rays that are nearly parallel to the line of sight. This is why side-view mirrors on cars are convex — to show more of the lane behind you.

Medium

Sunlight passes through a glass prism. Why do we see seven colours?

White light is a mixture of all colours. Each colour bends by a slightly different amount when entering and leaving the glass (because each has a different speed inside glass). After two refractions, the colours emerge at different angles and spread out into a spectrum.

Common Mistakes to Avoid

Measuring the angle from the surface instead of the normal: Always measure both angles (incidence and reflection) from the normal.
Confusing concave and convex: Concave = “caves in” (curved inward). Convex = bulges outward.
Saying “left-right inversion is because the mirror flips the image”: Actually, plane mirrors don’t flip left and right. They flip front-back. The “lateral inversion” we see is a consequence of looking at a 3D object from the opposite side.
Forgetting that the retinal image is inverted: The eye forms an upside-down image; the brain corrects the orientation. Babies’ brains learn this within months of birth.
Mixing up rods and cones: Rods → dim light, no colour. Cones → bright light, colour vision.

Practice Questions

Q1. State the two laws of reflection.

(i) The angle of incidence equals the angle of reflection. (ii) The incident ray, normal, and reflected ray all lie in the same plane.

Q2. Why is the road sign written in mirror-reverse on the front of an ambulance?

So that when a driver in front sees it in their rear-view mirror, the text appears correctly reversed (i.e., readable). Cars usually use mirrors to look behind them.

Q3. What kind of mirror is used in shaving mirrors? Why?

Concave mirror. When you hold your face close (within the focal length), it forms an enlarged, erect, virtual image — perfect for seeing fine details.

Q4. List the seven colours in white light in order from least to most bent in a prism.

Red, Orange, Yellow, Green, Blue, Indigo, Violet (least to most bent). The mnemonic VIBGYOR goes most-to-least.

Q5. What part of the eye gives it colour?

The iris.

Q6. What’s the difference between rods and cones?

Rods detect dim light but no colour. Cones detect bright light and colour. Three types of cones detect red, green, and blue.

Q7. A boy stands at 2 m from a plane mirror. He moves 1 m closer. How far apart are he and his image now?

New distance to mirror = 1 m. Image is 1 m behind mirror. Distance between him and image = 1 + 1 = 2 m.

Q8. What causes a rainbow?

Sunlight is dispersed by water droplets in the air. Each droplet acts like a tiny prism, splitting white light into its colours. Many droplets together produce the rainbow arc.

FAQs

Why can’t we see in complete darkness? No light reaches our eyes from objects, so the retina has no signal to send to the brain.

Why are dogs partially colour-blind? They have only two types of cones (mostly blue and yellow-green), while humans have three (red, green, blue). So dogs see fewer distinct colours.

Can light travel through a vacuum? Yes — sunlight reaches Earth from the Sun through the vacuum of space. (Sound, on the other hand, needs a medium and can’t travel in vacuum.)

Why does a swimming pool look shallower than it is? Because of refraction. Light from the bottom bends as it leaves the water, making the bottom appear closer than its true depth.

What’s the difference between reflection and refraction? Reflection: light bounces off a surface. Refraction: light passes through a surface and changes direction.

Are there colours we can’t see? Yes — ultraviolet (beyond violet) and infrared (beyond red). Some animals (bees, snakes) can detect these wavelengths even though humans can’t.