Light | Science Grade 8 CBSE

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The six ideas behind seeing

From a bouncing ray to a rainbow, and from a mirror image to the screen at the back of your eye — these six terms explain how we see. Tap each one to find out what it means.

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The three big ideas

Light travels in straight lines. When it strikes a surface and bounces back, that is reflection.
The ray going in is the incident ray; the line drawn perpendicular at the point it hits is the normal; the bounced ray is the reflected ray. The angle of incidence (i) and angle of reflection (r) are both measured from the normal.
Law 1: the angle of incidence equals the angle of reflection (i = r). Law 2: the incident ray, the reflected ray and the normal all lie in the same plane.
Regular reflection from a smooth, polished surface (a mirror, still water) gives a clear image. Diffused reflection from a rough surface (a wall, paper) scatters the rays, so no image forms — though each ray still obeys the laws.

Worked example. A ray hits a plane mirror making 30° with the mirror surface. What is the angle of reflection?

The angle is measured from the normal, not the surface. Angle of incidence = 90° − 30° = 60°. By the law of reflection, angle of reflection = 60°.

Common mistake: measuring the angle from the mirror surface. Always measure both angles from the normal (the perpendicular at the point of incidence).

A plane-mirror image is virtual (it cannot be caught on a screen) and erect (the right way up).
It is the same size as the object, and as far behind the mirror as the object is in front.
It is laterally inverted — left and right are swapped. That is why AMBULANCE is printed reversed on the front of the van, so a driver ahead reads it correctly in the mirror.
Two mirrors at an angle produce many images — the idea behind a kaleidoscope; a periscope uses two mirrors to see over obstacles.

Worked example. You stand 1 m in front of a plane mirror. Describe your image.

Your image appears 1 m behind the mirror, the same height as you, erect, facing you, with left and right swapped. It is virtual — you cannot project it onto a screen.

Dispersion: white light is really a mix of colours. A glass prism bends each colour by a different amount, spreading them into the spectrum — VIBGYOR (violet, indigo, blue, green, yellow, orange, red). A rainbow forms when raindrops act like tiny prisms.
The human eye: the cornea (front window) and lens focus light onto the retina at the back. The iris (the coloured ring) controls the pupil, which widens in dim light and narrows in bright light.
The image on the retina is real, inverted and smaller. Light-sensitive cells — rods (work in dim light, no colour) and cones (work in bright light, see colour) — send signals along the optic nerve to the brain, which turns the picture the right way up.
People who cannot see read and write using the Braille system of raised dots felt with the fingertips.

Worked example. Why do your pupils change size when you walk from a dark room into bright sunlight?

In the dark the iris opens the pupil wide to let in more light. In bright sunlight it makes the pupil small to limit the light and protect the retina.

Common mistake: thinking the retina sees an upright picture. The image on the retina is actually upside-down and smaller — the brain interprets it so the world looks the right way up.

Where you'll meet it

Light in everyday life

Mirrors on the road

Drivers use rear-view and side mirrors to see behind them. AMBULANCE is printed mirror-reversed on the bonnet so it reads correctly in another driver's mirror — lateral inversion put to work.

Rainbows after rain

When sunlight passes through millions of tiny raindrops, each drop disperses it into colours, and together they form a giant arc — the seven colours of VIBGYOR in the sky.

Seeing in dim light

Your eye's rod cells let you make out shapes in near-darkness, even when you cannot tell colours apart — which is why everything looks grey by moonlight.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and a case study, testing whether you can use the ideas, not just recall them.

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