Light: Shadows and ReflectionsClass 7 Science Notes

Light: Shadows and Reflections

11.1 Sources of Light

Light is a form of energy that allows us to see the world around us. Objects can either produce their own light or reflect light from other sources.

The Sun gives out or emits its own light and is the main source of natural light on the Earth. Other natural sources of light include stars, lightning, natural fire, and certain animals like fireflies.

In ancient times, humans learned to create fire, which was the earliest form of artificial lighting. They used various fuels like animal fat, oil, wax, and gas to create fire.

With the invention of electricity, different kinds of electric light sources were developed. Most of the lighting needs of humans are now met by electric lighting.

Objects are classified based on their ability to emit light

• Luminous objects emit their own light. Examples include the Sun, stars, and electric bulbs.
• Non-luminous objects do not emit their own light. They reflect light from luminous objects. The Moon is an example of a non-luminous object; it reflects the light emitted by the Sun.

Science and Society

Light Emitting Diode (LED) lamps are modern light sources that consume much less power, are brighter, and last longer than traditional lamps. Using LED lamps reduces electricity bills and is better for the environment. The Indian government promotes the use of LED lamps nationwide. At the end of their life, LED lamps must be disposed of or recycled appropriately, and not thrown in the garbage.

11.2 Does Light Travel in a Straight Line?

Light travels in a straight line. This property of light is also known as rectilinear propagation of light.

Light can sometimes even bend around corners, but this is a more advanced topic.

11.3 Light through Transparent, Translucent, and Opaque Materials

Materials can be classified based on how they interact with light

• Transparent materials allow light to pass through them almost completely. We can see clearly through transparent materials. Examples include glass and clear water.
• Translucent materials allow light to pass through them partially. We cannot see clearly through translucent materials. Examples include tracing paper and frosted glass.
• Opaque materials do not allow light to pass through them at all. We cannot see through opaque materials. Examples include cardboard, wood, and metal.

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

11.4 Shadow Formation

When an opaque object is placed in the path of light, it blocks the light. The dark patch formed behind the object, where light does not reach, is called a shadow.

Opaque objects form darker shadows, while translucent objects make lighter shadows. Even some transparent objects can create faint shadows.

To observe a shadow, we need a source of light, an opaque object, and a screen. The walls, floor, ground, or any other surface can act as a screen.

The shape, size, and sharpness of the shadow depend on the position of the object relative to the light source and the screen. The color of the shadow does not change when the color of the object is changed.

Fascinating Facts

Shadow play, or shadow puppetry, has been a part of our cultural heritage for centuries. In this art form, flat cut-out figures called shadow puppets are placed between a light source and a screen. By moving the puppets and the light, puppeteers can create life-like movements, bringing the characters to life. Different regions have their own unique styles, like the Charma Bahuli Natya in Maharashtra, Keelu Bomme and Tholu Bommalata of Andhra Pradesh, Togalu Gombeyaata in Karnataka, Ravana Chhaya in Odisha, Tholpavakoothu in Kerala, and Bommalattam in Tamil Nadu. These are used not only for entertainment but also communicate important messages to the community.

11.5 Reflection of Light

When light falls on a shiny surface, it bounces back. This phenomenon is called reflection of light.

A shiny surface or a mirror changes the direction of light that falls upon it.

11.6 Images Formed in a Plane Mirror

When we look into a mirror, we see a reflection of ourselves. This reflection is called an image.

The image formed by a plane mirror has the following characteristics

• It is of the same size as the object.
• It is upright, also called erect.
• It cannot be obtained on a screen.
• It is at the same distance behind the mirror as the object is in front of the mirror.
• It shows lateral inversion, where the left side of the object appears as the right side in the image, and vice versa.

11.7 Pinhole Camera

A pinhole camera is a simple device that uses a small hole to form an image. Light rays from an object pass through the pinhole and project an image on a screen.

The image formed by a pinhole camera is always inverted (upside down).

11.8 Making Some Useful Items

11.8.1 Periscope

A periscope is a device that allows us to see objects that are not directly visible. It uses two plane mirrors placed in a Z-shaped box. Reflection from the mirrors enables us to see objects that are not in our direct line of sight. Periscopes are used in submarines, tanks, or by soldiers to see outside their bunkers.

11.8.2 Kaleidoscope

A kaleidoscope is a device that creates beautiful patterns using mirrors and colored objects. It consists of three rectangular plane mirror strips joined together in a triangular manner. When we look through the kaleidoscope and turn it, we see different patterns formed by the multiple reflections of the colored objects. Designers and artists often use kaleidoscopes to get ideas for new patterns.