Light
ICSE Class 6 · Physics · Chapter 5
What this chapter covers
- Light travels in a straight line
- Applications of rectilinear propagation
- Pinhole camera
- Shadows
- Natural shadows: eclipses
Key points
- Light is a form of energy that makes objects visible to us.
- Light travels in straight lines; this is called rectilinear propagation of light.
- A ray is the straight path of light; a beam is a bundle of rays.
- Sunbeams, torch beams, aiming at a target, shadows and the pinhole camera all depend on light travelling in straight lines.
- A pinhole camera forms an inverted image with its left and right sides swapped.
- The image is bigger when the screen is farther from the pinhole, when the object is nearer the pinhole, or when the object is bigger.
- A shadow forms when an opaque object blocks light; it needs a light source, an opaque object and a surface.
- A shadow shows only the outline of an object, never its colour.
- The umbra is the fully dark centre of a shadow; the penumbra is the partly dark edge.
- A very small light source gives only an umbra; a large source gives an umbra and a penumbra.
- A lunar eclipse happens on a full-moon night, when the Earth's shadow falls on the Moon.
- A solar eclipse happens on a new-moon day, when the Moon's shadow falls on the Earth; never look at the Sun directly.
Questions answered
What is meant by rectilinear propagation of light?
Rectilinear propagation of light is the property of light by which it travels in straight lines.
Give two examples from daily life which show that light travels in a straight line.
A straight sunbeam entering a dark room through a gap, and the straight beam of a torch or headlight.
In the three-card activity, why does the candle flame disappear when the middle card is moved?
The holes are no longer in one straight line. Light from the flame cannot bend to pass through the moved hole, so it does not reach the eye.
On what principle does a pinhole camera work? Why is its image upside down?
It works on the rectilinear propagation of light. Rays from the top of the object travel straight through the pinhole and land at the bottom of the screen, and rays from the bottom land at the top. So the image is inverted.
Name three factors on which the size of the image in a pinhole camera depends.
The distance of the screen from the pinhole, the distance of the object from the pinhole and the size of the object.
The screen of a pinhole camera is moved from 8 cm to 16 cm behind the pinhole. The object is not moved. The first image was 3 cm tall. How tall is the new image?
The screen distance has doubled, so the image doubles too: 3 cm × 2 = 6 cm.
What three things are needed to form a shadow?
A source of light, an opaque object and a surface behind the object on which the shadow falls.
What is the difference between the umbra and the penumbra?
The umbra is the completely dark central part of a shadow, where no light reaches. The penumbra is the lighter outer part, where some light reaches.