What this chapter is about
This chapter explores how light travels and what happens when it meets different objects. You will learn why we can see some things clearly through glass but not through a wooden door. You will also discover how shadows form and why they change shape during the day.
Light is essential for seeing the world around us. Without light, our eyes cannot detect anything. This chapter helps you understand the basic behaviour of light, which you will use later to study mirrors, lenses and more advanced topics.
After studying this chapter, you should be able to explain why shadows form, classify materials based on how light passes through them, and understand how mirrors reflect light to form images.
Key ideas
- Light travels in straight lines. This is called rectilinear propagation of light.
- Objects that give out their own light are called luminous objects. Examples are the sun, a burning candle and a glowing bulb.
- Objects that do not give out their own light are called non-luminous objects. We see them only when light from a luminous source falls on them and bounces into our eyes.
- Materials through which light can pass completely are called transparent materials. Glass and clear water are transparent.
- Materials through which light cannot pass at all are called opaque materials. Wood, metal and stone are opaque.
- Materials through which light passes only partly are called translucent materials. Oiled paper, frosted glass and thin cloth are translucent.
- A shadow forms when an opaque object blocks the path of light. The shadow appears on the opposite side of the object from the light source.
- When light falls on a smooth, shiny surface like a mirror, it bounces back. This bouncing is called reflection of light.
Formulas and facts to remember
1. Luminous object – an object that produces its own light, like the sun or a torch.
2. Non-luminous object – an object that does not produce light; we see it by reflected light.
3. Transparent – allows almost all light to pass through; objects behind it are clearly visible.
4. Translucent – allows some light to pass; objects behind it appear blurry.
5. Opaque – does not allow any light to pass; a shadow forms behind it.
6. Shadow – a dark region formed when an opaque object blocks light.
7. Reflection – the bouncing back of light from a surface.
8. Image – what you see when light reflects from a mirror; it looks like the object but is not the object itself.
Worked examples
Example 1: Classifying materials
Ravi holds three items in sunlight: a clear glass tumbler, a sheet of cardboard and a piece of butter paper. Classify each.
Solution
- Clear glass tumbler: Light passes through completely, so it is transparent.
- Cardboard: No light passes through, so it is opaque.
- Butter paper: Some light passes, but you cannot see clearly through it, so it is translucent.
Example 2: Shadow size
Priya stands 2 metres from a wall at night. A streetlight is behind her, 4 metres from the wall. Will her shadow be taller or shorter than her height?
Solution
Light travels in straight lines from the streetlight. Because Priya is closer to the wall than the light source, her shadow will be shorter than her actual height. If she moves closer to the light and farther from the wall, the shadow will become larger.
Example 3: Mirror reflection
Ankit writes the number 3 on paper and holds it in front of a plane mirror. What does he see?
Solution
A plane mirror reverses the image from left to right. The number 3 will appear reversed, looking like the letter E (a backward 3). This is called lateral inversion.
Common mistakes
Thinking all shiny objects are mirrors → A shiny metal spoon is curved, so it reflects light differently from a flat mirror; not all shiny surfaces give a clear image.
Believing shadows have colour → Shadows are simply regions where light is blocked; they do not have colour of their own.
Confusing translucent with transparent → If you can read a book through the material, it is transparent; if you see only blurred shapes, it is translucent.
Thinking the shadow always equals the object's size → Shadow size depends on the positions of the light source, the object and the screen.
Assuming light can bend around corners easily → Light travels in straight lines; it does not curve around opaque objects, which is why shadows have sharp edges.
Quick revision
- Light travels in straight lines; this explains shadow formation.
- Luminous objects make their own light; non-luminous objects only reflect light.
- Transparent lets all light through, translucent lets some, opaque lets none.
- Shadows form on the side opposite the light source.
- Reflection is light bouncing off a surface; a plane mirror shows a laterally inverted image.