Lens
Category : 10th Class
A transparent object having two refracting surface is called lens. It is of two types, concave lens and convex lens. Working of lens is based on the refraction of light passing through it.
Convex Lens
Convex lens is the lens which is thicker at the center and thinner at the edge. It has two refracting surfaces. It is also called the converging lens as the ray of light converges at a point after refraction. The point, at which ray of light converges after refraction, is called the focus of the lens.
Concave Lens
It is the lens which is thinner at the center and thicker at the edge. It also has two refracting surfaces. It is called diverging lens as the ray of light diverges in different direction after refraction.
The centre of the lens is called the optical center. It is normally denoted by C. The ray of light passing through optical center do not undergo any’ deviation, and goes in a straight line. Thus, the line passing through the optical center of the lens and perpendicular to both the face of the lens, is called the principal axis. The ray of light that converges at a point on the principal axis is known as the principal focus. The distance between focus and optical centre is called the focal length.
Image Formation by a Convex Lens
I. When the object is at infinity
Properties of Image formed is
II. When the object is beyond \[2{{F}_{1}}\]
Properties of Image formed is
III. When the object is at \[2{{F}_{1}}\]
Properties of Image formed is
Properties of Image formed is
V. When the object is at \[{{F}_{1}}\]
Properties of Image formed is
VI. When the object is \[{{F}_{1}}\]is between optical centre
Properties of Image formed is
The summary of the above observation is given in the table below:
S. No. | Position of Object | Position of Image | Nature of Image | Size of Image |
1. 2. 3. 4. 5. 6. |
At infinity Beyond 2F1 At 2F1 Between F1 and 2F1 At F1 Between F1 and O |
At Focus Between F2 and 2F2 At 2F1 Beyond 2F2 At Infinity Behind Object |
Real and Inverted Real and Inverted Real and Inverted Real and Inverted Real and Inverted Virtual and Erect |
Highly diminish Smaller in size Equal in size Equal in size Highly Enlarge Larger in size |
Image Formation by a Concave Lens
When the objects at any point between optical center and infinity
Properties of Image
A. Image is formed between 0 and P.
B. Image is Virtual and Erect.
C. Smaller is size.
Uses of Concave Lens
Uses of Convex Lens
Sign Convention used in Lens
Lens Formula
The mathematical relationship between the object distance, image distance, and focal length is called the lens formula. It is expressed as;
\[\frac{1}{f}=\frac{1}{v}=\frac{1}{u},\]where, f is the focal length, v is the image distance and u is the object distance.
Magnification of the lens is defined as the ratio of height of image to theheight of object or ratio of image distance to the object distance. It is expressed as:
\[m=\frac{v}{u}=\frac{{{h}_{2}}}{{{h}_{1}}}\]
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