Light
Overview
A mirror shows us our face, a torch lights up a mountain, a rainbow displays 7 colours — all of these are the work of light. In this chapter, we will learn about reflection, mirrors, lenses, and the 7 colours of the rainbow.
Typed Notes
Laws of Reflection
When light falls on a mirror, it bounces back — this is called reflection. The laws are: (1) The angle of incidence (the angle at which light falls, measured from the normal) equals the angle of reflection (the angle at which light bounces back); (2) The incident ray, the reflected ray, and the normal all lie in the same plane. The normal is an imaginary line drawn perpendicular to the mirror at the point where light hits it. This is why if you shine a torch beam straight at a mirror, it reflects straight back. The same principle is used in a periscope — two mirrors placed at 45° — which allows us to see over or around a wall.
4 Properties of a Plane Mirror Image
The image formed in a plane (flat) mirror has four properties: (1) Virtual — it is not real; it appears to form behind the mirror and cannot be caught on a screen; (2) Erect — the image is upright, not inverted; (3) Same size — the image is the same size as the object; (4) Laterally inverted — left and right are swapped (your right side appears as the left side in the image — this is why the word 'AMBULANCE' is written in reverse on the vehicle, so that it reads correctly in a driver's rear-view mirror). The image appears as far behind the mirror as the object is in front of it.
Regular vs. Diffuse Reflection
Regular reflection — smooth surface (mirror, still water): all parallel rays reflect in the same parallel direction — in one direction only — which is why you can see a clear image of your face in a mirror. Irregular or diffuse reflection — rough surface (wall, paper, leaf): rays scatter in all directions — which is why we can see every object from any direction (a wall can be seen from any angle). Diffuse reflection is not bad — it is essential: without it, only mirrors would be visible to us, and nothing else in the world would be seen!
Mirrors, Lenses, and 7 Colours
Concave mirror (curved inward, converging): used as the reflector in torches and headlights (the filament is placed at the focus — all rays emerge as a parallel beam that travels far), and as a shaving mirror (when the face is held close, it gives a magnified image). Convex mirror (curved outward, diverging): used as a rear-view mirror in vehicles — it gives a wide field of view and a small, erect image (so the driver can see many vehicles behind at once). Convex lens (thicker in the middle, converging): used in a magnifying glass (held close — gives a larger image), camera, the human eye, and telescopes. Prism: splits white light into 7 colours (Violet-Indigo-Blue-Green-Yellow-Orange-Red) — this is called dispersion — each colour bends by a different amount. Rainbow: raindrops act like tiny prisms — they split the Sun's white light into 7 colours — that is what a rainbow is.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Light travels in a straight line; Reflection: light hits a mirror and bounces back — angle of incidence = angle of reflection; both rays lie in the same plane.
- 02Plane mirror: image is virtual (forms behind the mirror), erect (upright), same size as the object, and laterally inverted (left and right are swapped).
- 03Regular reflection — smooth surface (mirror) — one direction; Irregular/diffuse — rough surface (wall, paper) — all directions — that is why we can see everything around us.
- 04Concave mirror (curved inward) — converging: used in torches and headlights (produces parallel beams), and as a shaving mirror (gives a magnified image of the face). Convex mirror (curved outward) — diverging: used as a rear-view mirror (wide field of view, small erect image).
- 05Concave lens — diverging; Convex lens — converging (used in magnifying glass, camera, eye, telescope). Prism — splits white light into 7 colours (VIBGYOR) — dispersion; the rainbow is a natural example of this.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
Write the two laws of reflection.
Write the 4 properties of the image formed in a plane mirror.
Why can we see a wall from every direction, but a mirror only from one specific direction?
Why are rear-view mirrors in vehicles convex?
How does a prism split white light into 7 colours?
Why is the reflector inside a torch concave?
Reflection: angle of incidence = angle of reflection, all in one plane. Plane mirror image: virtual, erect, same size, laterally inverted. Regular reflection (smooth surface) vs. diffuse reflection (rough surface — visible from all directions). Concave mirror: used in torches and as shaving mirror; convex mirror: used as rear-view mirror. Convex lens: used for magnification and in cameras. Prism: produces 7 colours (VIBGYOR) — dispersion — the basis of the rainbow.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Why does the image in a plane mirror not appear on a screen?
Q2.Why is 'AMBULANCE' written in reverse on the vehicle?
Q3.For what purpose is a convex mirror used?
Q4.White light is a mixture of how many colours?
Mark karo — dashboard par progress dikhega.