Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Light travels in a straight line. This property is known as the Rectilinear Propagation of Light.
A shadow is a dark region formed when an opaque object comes in the path of light rays.
There are three essential requirements for shadow formation: (1) A source of light, (2) An opaque object to obstruct the light, and (3) A screen or surface where the shadow can be formed.
Opaque objects (like wood or stone) form dark, distinct shadows. Translucent objects (like butter paper) form faint shadows, and transparent objects (like clear glass) do not form shadows because light passes through them.
The color of a shadow is always black or grey, regardless of the color of the object used to cast it.
The size of a shadow changes depending on the distance between the light source and the object, and the distance between the object and the screen.
Natural shadows: During the day, the length of shadows changes. Shadows are longest in the early morning and evening (when the sun is at a low angle) and shortest at noon (when the sun is directly overhead).
📐Formulae
💡Examples
Problem 1:
A girl is standing in the sun at noon. What will be the characteristic of her shadow compared to its state at AM?
Solution:
The shadow will be at its shortest length at noon.
Explanation:
At noon, the sun is positioned directly overhead. The light rays fall vertically on the object, resulting in the minimum area of light being blocked and projected onto the ground. At AM, the sun is at a lower angle, casting longer, slanted shadows.
Problem 2:
If a torch is moved closer to a toy placed in front of a wall, what happens to the size of the shadow?
Solution:
The size of the shadow on the wall will increase.
Explanation:
When the distance between the source of light and the object () decreases, the object blocks a larger angle of light rays originating from the source, causing the shadow projected on the screen to become larger.
Problem 3:
Why do birds flying high in the sky not cast a shadow on the ground?
Solution:
The bird is too far from the screen (the ground).
Explanation:
As the distance between the object and the screen increases, the shadow becomes blurred and eventually disappears. For a bird high in the sky, the is so large that the light rays bend around the bird (diffraction/dispersion effects) or the shadow is so faint that it is not visible.