Maharashtra Board Class 9 Science Chapter 11 Reflection of Light PDF Download

Reflection of Light

Can You Recall?

1. What is light?

2. What is meant by reflection of light? Which are the different types of reflection?

Light is an agent which gives us information about what is happening in our surroundings. We can enjoy various wonders of nature like the sunrise, sunset and rainbow only because there is light. It is only because of light that we can see the lush green vegetation, colourful flowers, deep blue skies in the day, stars shining in the dark night sky in the beautiful world around us. We can see the man-made objects in our surroundings as well. Light is actually electromagnetic radiation which causes the sensation of vision.

Light reflects differently from the various surfaces around us. We have already learnt that the reflection from smooth, flat surfaces is regular reflection while rough surfaces reflect light irregularly.

We need polished surfaces for reflecting light. This is because polished surfaces absorb less light and so maximum light gets reflected.

In scientific language, a surface which reflects light and creates clear images is called a mirror. A mirror is a reflecting surface.

We use various types of mirrors in our daily life. Mirrors are of two types: plane mirrors and spherical mirrors.

Teacher's Note

When you look in a bathroom mirror at home, you see a clear image of yourself. This is a plane mirror. It is made by putting silver coating on the back of flat glass.

Exam Trick

Remember: Plane mirror = flat and smooth. Spherical mirror = curved like a ball. The shape of the mirror changes the type of image you see.

Points to Remember

Light is what helps us see things around us.
A mirror is a smooth surface that reflects light and makes clear images.
There are two types of mirrors: plane and spherical.
Polished surfaces reflect more light than rough surfaces.

Plane Mirror

Can You Tell?

What is a mirror?

A mirror is made by coating the back surface of a flat and smooth glass piece with a thin, reflecting film of aluminium or silver. To protect this reflecting film and to make that side opaque, another coat of a substance like lead oxide is given over it.

An Introduction To Scientists

The German scientist Justus von Liebig coated the plane surface of a piece of ordinary glass with silver metal and made the first mirror. Such a mirror is called a silvered glass mirror.

Teacher's Note

The mirrors in your home or school are plane mirrors. They are made by coating glass with silver at the back. This silver coating reflects light to show your image.

Exam Trick

Remember: Silvered glass mirror = first mirror made by Justus von Liebig. Just remember the name and that it was silver coated.

Points to Remember

A plane mirror is made of flat glass with silver coating on the back.
The silver coating protects the glass and helps reflect light.
Lead oxide is used to make the back opaque and strong.

Formation Of Images By A Mirror

Can You Tell?

1. If we hold a page of a book in front of a mirror, we see laterally inverted letters in the mirror. Why does it happen?

2. Which letters of the English alphabet form images that look the same as the original letters?

We see a clear image of ourselves when we stand in front of a mirror in our house. To understand how an image is formed in a mirror let us first consider an image formed by a point source. Light rays travel in all directions from such a source. Several of these rays fall on the mirror, get reflected and reach our eyes. Due to reflection, they appear to be coming from a point behind the mirror. That point is the image of the point source.

Light rays falling perpendicularly on the mirror are reflected back in the perpendicular direction.

A point source O in front of the plane mirror is placed. Incident rays get reflected according to the laws of reflection. When these reflected rays are extended behind the mirror, they meet at a point. When seen from the eye position, the rays appear to be coming from that point. Other rays starting from the source also get reflected and appear to be coming from the same point. Thus, that point is the image of the source.

The reflected rays do not actually meet. Hence, such an image is called a virtual image. The perpendicular distance of the image from the mirror is equal to the perpendicular distance of the source from the mirror.

If we use an extended source instead of a point source, an image is formed of every point of the source, thereby forming an extended image of the whole source. An extended source is kept in front of the mirror. The images of different points are formed at different positions. Similarly, images of all points between two points are formed between their image positions resulting in the image of the entire extended source.

The image formed by a plane mirror is of the same size as the source.

The image of a word appears laterally inverted in the mirror. The image of every point on the word is formed behind the mirror at the same distance from the mirror as the point itself. This is called lateral inversion.

Teacher's Note

When you write something and hold it in front of a mirror, the words look backwards. This is called lateral inversion. The image is not real - it is just light rays reflecting back to your eyes.

Exam Trick

Remember: Virtual image = behind the mirror. Real image = in front of the mirror on a screen. Lateral inversion = left becomes right in the mirror image.

Points to Remember

Images in plane mirrors are virtual - they cannot be seen on a screen.
The image distance equals the object distance from the mirror.
The image is the same size as the object.
Lateral inversion means left and right appear reversed in the mirror.

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