Light: Shadows and Reflections

Class 07 Science

The Sun gives out or emits its own light and is the main source of natural light on the Earth. Stars, lightning, natural fire, and certain animals also emit their own light.

In ancient times, humans learnt to create fire - the earliest form of artificial lighting. With time, they learnt to create fire using different fuels, such as animal fat, oil, wax, and gas.

With the invention of electricity and different kinds of electric light sources, most of the lighting needs of humans are now met by electric lighting.

Objects that emit their own light are called luminous objects. Objects that do not emit their own light are called non-luminous objects. The Moon is a non-luminous object. It does not emit its own light. It just reflects the light emitted by the Sun that falls on it.

Light travels in a straight line.

Light passes almost completely through transparent materials. Light passes partially through translucent materials. Light does not pass through opaque materials.

Shadow

Light travels in a straight line. So, when an opaque object is placed in its path, light is blocked. The dark patch, where light does not reach, is the shadow. Translucent objects make lighter shadows.

Shadows are formed when an object blocks light from falling on a screen. We need a source of light, an opaque object, and a screen to observe a shadow. The walls, floor, ground, or any other surface acts as a screen for observing shadows in our daily life.

The shape, size, and sharpness of the shadow depend on the position of the object relative to the light source and the screen. The shadows may give information about the object or we may not be able to guess the object at all. Changing the colour of opaque objects does not change the colour of the shadows.

Reflection of Light

A shiny surface or a mirror changes the direction of light that falls upon it. This change in direction of light by a mirror is called the reflection of light.

Images Formed in a Plane Mirror

1. The image formed by a plane mirror is of the same size as the object.
2. An upright image is called erect. An image formed by a plane mirror is erect.
3. The image formed by a plane mirror cannot be obtained on a screen.
4. There is lateral inversion in the images formed by a plane mirror. The perceived left-right reversal is called lateral inversion.

The image formed by a plane mirror is of the same size as the object, is erect, cannot be obtained on a screen, and is laterally inverted.

Pinhole Camera

A pinhole camera is a device in which the light rays from an object pass through a tiny hole (a pinhole) and form an image on a screen.

Periscope

Reflection from two mirrors enables to see objects that are not visible directly. Periscopes are used in submarines, tanks, or by soldiers to see outside their bunkers.

Kaleidoscope

Get three rectangular plane mirror strips of equal width and join them together in a triangular manner.

An interesting feature of the kaleidoscope is that one always gets to see a different pattern every time the kaleidoscope is turned about. Since there are 3 mirrors, and multiple images (due to reflections of reflections), many interesting patterns are formed. Designers and artists often use kaleidoscopes to get ideas for new patterns.