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TS Polycet (Polytechnic) 2022 Previous Question Paper with Answers And Model Papers With Complete Analysis

71) What is the focal length of a plano-convex lens if ‘R’ is the radius of curvature and ‘n’ is the refractive index?

A) f=R

B) $f=\frac R2$

C) $f=\frac R{n-1}$

D) $f=\frac{n-1}R$

View Answer

C) $f=\frac R{n-1}$
Explanation:Plano-convex lens focal length:
$f = \frac{R}{n - 1}$

72) What is lens maker’s formula?

A) $\frac1f=\left(n-1\right)\left(\frac1{R_1}+\frac1{R_2}\right)$

B) $\frac1f=\left(n-1\right)\left(\frac1{R_1}-\frac1{R_2}\right)$

C) $\frac1f=\left(n+1\right)\left(\frac1{R_1}+\frac1{R_2}\right)$

D) $\frac1f=\left(n-1\right)\left(\frac1{R_1}-\frac1{R_2}\right)$

View Answer

D) $\frac1f=\left(n-1\right)\left(\frac1{R_1}-\frac1{R_2}\right)$
Explanation:Lens maker’s formula is:
$\frac{1}{f} = (n - 1)\left( \frac{1}{R_1} - \frac{1}{R_2} \right)$

73) The focal length of a lens depends on

A) (1) Radius of curvature

B) (2) Refractive index of the lens

C) (1) and (2)

D) None of the above

View Answer

C) (1) and (2)
Explanation:Focal length depends on both radius of curvature and refractive index

74) The size of the image formed by a convex mirror is always

A) Enlarged

B) Diminished

C) Double size of the object

D) None

View Answer

B) Diminished
Explanation:Convex mirror always gives diminished image
Diminished

75) The spherical mirror whose reflecting surface curved outward is

A) Convex mirror

B) Concave mirror

C) Plane mirror

D) None

View Answer

A) Convex mirror
Explanation:Reflecting surface curved outward = convex mirror

76) Every lens has ______ focal points

A) 2

B) 4

C) 6

D) 8

View Answer

A) 2
Explanation:Every lens has 2 focal points (on both sides)

77) If a convex lens is placed in water, it’s focal length

A) Increases

B) Decreases

C) Does not change

D) None of the above

View Answer

A) Increases
Explanation:In water, refractive index contrast decreases ⇒ focal length increases
Increases

78) The type of mirror used in solar cooker is

A) Concave

B) Convex

C) Concave and Convex

D) None

View Answer

A) Concave
Explanation:Concave mirror is used in solar cookers to focus sunlight
Concave

79) The minimum distance of an object from the pole to obtain real image in case of a concave mirror is

A) F

B) 2F

C) 0

D) $\frac F2$

View Answer

B) 2F
Explanation:For real image in concave mirror, object should be beyond F, so minimum is 2F

80) If the focal length of a spherical mirror is 10cm, what is the value of radius of curvature?

A) 10cm

B) 20cm

C) 30cm

D) None

View Answer

B) 20cm
Explanation: $f = \frac{R}{2} \Rightarrow R = 2f = 20cm$

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