10th Class Science Light - Reflection and Refraction Power of Lens

Power of Lens

Category : 10th Class

*        Power of Lens


Power of the lens is defined as the ability of the lens to converge or diverge the ray of light passing through it. The power of the convex lens is said to be more, if the ray of the light converges more strongly towards the optical center. The power of the lens depends on the focal length of the lens. It is defined as the reciprocal of the focal length in meters. It is expressed as \[P=\frac{1}{f}\].



The SI unit of power of the lens is diopter and is denoted by D. One diopter is defined as the power of the lens having focal length of one meter. The focal length of the convex lens is positive so the power is positive and that of the concave lens is negative as its focal length is negative.

Types of Simple Lenses



*            Power of Combination of Lens

When number of lens are placed in contact with each other, the power of combination of the lenses is equal to the algebraic sum of the powers of the individual lenses. If \[{{P}_{1}},\,\,{{P}_{2}},\,\,{{P}_{3}},\,---\,{{P}_{n}}\] are n lenses placed in contact with each other than the combined power of the lens is given by \[P={{P}_{1}}+{{P}_{2}}+{{P}_{3}}+---+{{P}_{n}}\]

The system of combination of lens is used in designing the optical instrumentlike cameras, microscope and telescope. It increases the sharpness of the image.





         The speed of light in water is found to be:

(a) \[3\times {{10}^{8}}\,m/s\]                 

(b) \[3.2\times {{10}^{8}}\,m/s\]

(c) \[1.2\times {{10}^{8}}\,m/s\]

(d) \[2.25\times {{10}^{8}}\,m/s\]

(e) None of these


Answer: (d)



         A stick partially immersed in water appears to be bent when seen from outside because of:

(a) Reflection

(b) Refraction

(c) Dispersion

(d) Scattering

(e) None of these


Answer: (b)



           On which one of the following does the refractive index of the light depend?

(a) Intensity

(b) Frequency

(c) Wavelength

(d) Density of medium

(e) None of these


Answer: (c)



         Where will the image forms if the object is placed at 2F in case of convex lens?

(a) At F

(b) Between F and 2F

(c) At 2F

(d) Beyond 2F

(e) None of these


Answer (c)



        Find the power of lens whose focal length is +10 cm.

(a) -10 D

(b) +10 D

(c) +1D

(d) -1 D

(e) None of these


Answer: (b)

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