Light allows us to see the world around us. In this chapter we explore how light travels, what shadows are, how mirrors reflect light, and the properties of different types of images formed by mirrors.
Key Concepts and Definitions
Light travels in straight lines (rectilinear propagation). This is demonstrated by the fact that a small hole (pinhole) in an opaque box creates an inverted image of an object outside.
Luminous objects produce their own light (e.g., the Sun, a burning candle, electric bulb). Non-luminous objects do not produce their own light; they are visible only when light from a luminous source falls on them and is reflected to our eyes (e.g., the Moon, a book).
Transparent materials allow almost all light to pass through (e.g., clear glass, clean water). Translucent materials allow only some light to pass (e.g., tracing paper, frosted glass). Opaque materials allow no light to pass through (e.g., wood, brick).
Shadow is a dark region formed when an opaque object blocks light. Shadows form on the side of the object away from the light source. The size of a shadow depends on the distance of the object from the light source and from the screen.
- 1.Reflection of light: When light falls on a polished surface (mirror), it bounces back. The law of reflection states:
- 2.The angle of incidence = the angle of reflection.
- 3.The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
- Plane mirror: A flat mirror. The image formed is:
- Virtual (cannot be caught on a screen)
- Erect (upright)
- Laterally inverted (left-right reversed)
- Same size as the object
- As far behind the mirror as the object is in front
Multiple reflections: Two plane mirrors facing each other at an angle produce multiple images. The number of images formed = (360/angle) - 1 (when angle divides 360 evenly).
Worked Examples
Why does a pinhole camera produce an inverted image?
Light travels in straight lines. Rays from the top of an object pass through the pinhole and travel downward to hit the lower part of the screen; rays from the bottom travel upward. The image is therefore upside down (inverted).
A candle is 10 cm in front of a plane mirror. How far is the image from the mirror? What are its characteristics?
The image is 10 cm behind the mirror (same distance as the object). It is virtual, erect, same size as the candle, and laterally inverted.
A ray of light hits a plane mirror at an angle of incidence of 30°. What is the angle of reflection?
By the law of reflection, angle of reflection = angle of incidence = 30°. The total angle between the incident ray and reflected ray = 60°.
Two mirrors are placed at 90° to each other. How many images are formed of a pencil placed between them?
Number of images = (360/90) - 1 = 4 - 1 = 3 images.
Why does a shadow become shorter as the Sun rises higher?
When the Sun is low (morning), its rays are more oblique, casting long shadows. As the Sun rises, rays become more vertical, and shadows shorten. At noon the shadow is shortest (Sun overhead).
A torch is shone on a ball from very close. What kind of shadow forms?
A large shadow forms because the ball blocks a wide cone of light. If the torch is moved farther away, the shadow becomes smaller and sharper.
Why can you see your face in a calm lake but not in rough water?
A calm lake surface acts like a plane mirror — it is smooth and produces regular (specular) reflection, giving a clear image. Rough water scatters light in all directions (diffuse reflection) so no clear image is formed.
- Key Facts
- Speed of light in vacuum: approximately 3 x 108 m/s (3 lakh km/s).
- Angle of incidence and reflection are measured from the normal (perpendicular to the mirror surface), not from the mirror surface itself.
- Lateral inversion: text appears mirror-reversed — the word AMBULANCE is written in reverse on ambulances so drivers can read it correctly in their rear-view mirrors.
Common mistakes
Students often measure angles from the mirror surface instead of from the normal. Always draw the normal (perpendicular) first. Also, a virtual image is not a "fake" image — it is a real perception; it simply cannot be projected onto a screen.
Summary
Light travels in straight lines. Objects that produce light are luminous; others are non-luminous. Opaque objects form shadows. Mirrors reflect light according to the law of reflection (angle of incidence = angle of reflection). A plane mirror forms a virtual, erect, same-size, laterally inverted image as far behind the mirror as the object is in front.