# 7th Class Science Lights and Sound Lenses and Formation of Image

Lenses and Formation of Image

Category : 7th Class

### Lenses and Formation of Image

There are two types of lens:

• Convex
• Concave

The middle portion of the convex lens is thicker than its edges and the middle portion of the concave mirror is thinner than its edges. The images formed by convex lenses are: (a) real and inverted; (b) virtual and erect. The formation of image by a convex lens is very similar to the formation of image by concave mirror.

Look at the following pictures of lenses  Fig. (1) Side view of convex lens                                     Fig. (1) Front view of convex lens  Fig. (3) Side view of concave lens                                    Fig. (3) Front view of concave lens

Lenses are made up of spherical mirrors. The same thickness of the both curvature causes zero optical power. Two surface of mirror combined and forms a lens. The surfaces of the lens are ideally perpendicular to each other. The surface of the lens can be of plane, or of spherical mirrors. The joining line between the vertexes of the both surface of the lens is called its axis. Convex Lens

In the figure below/is the focal length, a is the distance between the two surfaces of the lens. R is the radius of curvature of the first surface of the lens and R^ is the radius of curvature of the secondary lens.

Look at the following picture of image formation by a convex lens  Formation of Image by a Convex Lens

When the position of the object before a convex lens is changed, nature of the image formed by the lens is also changed. Formation of image by a convex lens when object is at the different position in front of the lens and characteristics of the image has been given below.

 Position of object Position of image Size of image Nature of image 1. Between f and optical center (c) On the same side as object Enlarged Virtual and erect 2. At f At infinity Highly enlarged Real and inverted 3. Between f and 2f Beyond 2f Enlarged Real and inverted 4. At 2f At 2f Same size as object Real and inverted 5. Beyond 2f Between f and 2f Diminished Real and inverted 6. At infinity At f Highly diminished Real and inverted Concave Lens

Optical centre and principal axis of a concave lens has been shown below.

Look at the following picture of the concave lens  Formation of Image by a concave lens

There are two types of images. These are formed when an object is placed in front of a concave mirror. Formation of image by a concave lens when object is at the different position in front of the lens and characteristics of the image has been given below.

 Position of object Position of image Size of image Nature of image 1. Anywhere between optical center C and Infinity Between optical center (c) and focus (F) Diminished Virtual and erect 2. At infinity At focus (F) Highly diminished Virtual and erect  There are two curvatures found in a convex lens. Which one of the following causes the zero optical power of the lens?

(a) Equal thickness of both the curvatures

(b) Unequal thickness of both the curvature

(c) Distance between the focal point and center of the curvature

(d) All of these

(e) None of these

Answer: (a) Rays of light from an object passes through a convex lens. The image is formed at focus behind the incoming light. Which one of the following lens is used for the same?

(a) Concave lens

(b) Diverging lens

(c) Converging lens

(d) All of these

(e) None of these

Answer: (c) Dispersion and Refraction of Light

When a glass prism is placed in the path of a beam of light, the beam of light splits into seven colours, like a rainbow (violet, indigo, blue, green, yellow, orange and red) (VIBGYOR) and this process is known as dispersion of light. The splitting of single beam of light into seven colours is called spectrum. This experiment was conducted by Newton and concluded that a white beam of light consists of seven colours.

Look at the following picture of splitting of light through a glass prism Fig. (1) A glass prism Fig. (2) A compact disk

In the picture (1), a beam of white light passes through a glass prism and splitting into seven colours. From the above experiment it is clear that a beam of light consists of seven colours and it splits into its constituent colours when a glass prism is placed in the path of ray of light. A compact disc is splitting a beam of light into seven colours as shown in the figure (2). Refraction of Light

A beam of light travels in various transparent medium like, in water, glass, liquid, etc. Speed of light gets changed when it enters from one medium to another. The speed of light and angle of inclination in a transparent medium depends on the refractive index of the medium. Different materials have different refractive index.

Look at the following picture of the refraction of light In the picture above a beam of light passes through two medium and a ray of light bends when enters into another medium. The angle of inclination with respect to the normal is due to the refractive index of the medium. The angle between the refracted ray and normal is called angle of refraction. Angle of refraction in another medium gets changed according to the refractive index of the medium. When light is passing through a medium and entering into the other medium then according to snells law,

$Ni\times Sin\,(Ai)=Nr\times Sin\,(Ar)$

Where, Ni is the refractive index of the medium from where light is entered. $Ai$is the angle between the incidence ray and normal (a perpendicular line exactly on the boundaries of both the medium). Nr is the refractive index of the medium in which light is entered. $Ar$is the refractive angle between the refracted ray and the normal. The refractive index of vacuum is 1.  A ray of light consists of seven colours as we can see in a rainbow.

How can it be easily shown in the physics lab?

(a) When a ray of light falls on a transparent glass prism

(b) When a ray of light falls on a plane mirror

(c) When a ray of light falls on a spherical mirror

(d) All of these

(e) None of these

Answer: (a) A beam of light bends when passes from one medium to another.

Which one of the following is responsible for the bending of ray of light?

(a) Cross sectional area of the medium

(b) Total area of the medium

(c) Refractive index of the medium

(d) All of these

(e) None of these

Answer: (c) Some objects produce light and are called luminous objects.

Some objects do not produce light and are called non-luminous object.

The formation of image of an object by the mirror is possible due to the reflection of light.

The objects that allow passing of light through them are called transparent objects.

Light travels in straight path and this property of light is called rectilinear propagation of light.

Opaque objects do not allow passing of light through them.

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