NEET Ray Optics — Set 5

Question 1

A convex lens of power $ +4 \, \text{D} $ is used. What is its focal length?

Accepted Answer

Power: $ P = \frac{1}{f} $ (in meters). $ P = +4 \, \text{D} \Rightarrow 4 = \frac{1}{f} \Rightarrow f = \frac{1}{4} = 0.25 \, \text{m} = 25 \, \text{cm} $.

Question 2

A ray of light passes from glass ($ n = 1.62 $) to water ($ n = 1.33 $) at an angle of incidence of $ 50^\circ $. What is the angle of refraction?

Accepted Answer

Snell’s law: $ n_1 \sin i = n_2 \sin r $. Glass ($ n_1 = 1.62 $), water ($ n_2 = 1.33 $), $ i = 50^\circ $. $ 1.62 \times \sin 50^\circ = 1.33 \times \sin r $. $ \sin 50^\circ \approx 0.766 \Rightarrow 1.62 \times 0.766 \approx 1.241 \Rightarrow 1.33 \sin r = 1.241 $. $ \sin r = \frac{1.241}{1.33} \approx 0.933 \Rightarrow r = \sin^{-1}(0.933) \approx 68.9^\circ $.

Question 3

Why does a convex lens submerged in a liquid with the same refractive index as the lens not form an image?

Accepted Answer

When a convex lens is in a medium with the same refractive index, there is no difference in refractive indices between the lens and the medium. Without this difference, light does not bend at the interfaces, and the lens acts like a flat plate, failing to converge or diverge rays to form an image.

NEET Physics: Ray Optics — Practice Set 5

Q1. A convex lens of power \( +4 \, \text{D} \) is used. What is its focal length?

Q2. A ray of light passes from glass (\( n = 1.62 \)) to water (\( n = 1.33 \)) at an angle of incidence of \( 50^\circ \). What is the angle of refraction?

Q3. Why does a convex lens submerged in a liquid with the same refractive index as the lens not form an image?

Q4. A compound microscope has an objective of focal length \( 1.5 \, \text{cm} \) and eyepiece of focal length \( 5 \, \text{cm} \) with a tube length of \( 20 \, \text{cm} \). What is the magnification at infinity?

Q5. In a convex lens submerged in a medium of higher refractive index, what happens to its ability to focus light?

Q6. An object of height \( 2 \, \text{cm} \) is placed \( 10 \, \text{cm} \) from a concave mirror of focal length \( 5 \, \text{cm} \). What is the height of the image?

Q7. A prism of angle \( 30^\circ \) has a minimum deviation of \( 20^\circ \). What is the refractive index?

Q8. Why does a convex lens fail to form a sharp image when used with white light unless corrected?

Q9. A double convex lens of refractive index \( 1.5 \) has radii of curvature \( 40 \, \text{cm} \) and \( -40 \, \text{cm} \). What is its focal length?

Q10. A double convex lens of refractive index \( 1.55 \) has radii of curvature \( 30 \, \text{cm} \) and \( -30 \, \text{cm} \). What is its focal length?

Q11. A converging beam meets a convex lens (\( f = 12 \, \text{cm} \)) \( 6 \, \text{cm} \) before the convergence point. What is the new image distance?

Q12. A ray of light passes from air (\( n = 1 \)) to water (\( n = 1.33 \)) at an angle of incidence of \( 50^\circ \). What is the angle of refraction?

Q13. A convex lens (\( f = 50 \, \text{cm} \)) and a concave lens (\( f = 25 \, \text{cm} \)) are in contact. What is the effective focal length?

Q14. An object is placed \( 12 \, \text{cm} \) from a concave mirror of focal length \( 6 \, \text{cm} \). What is the image distance?

Q15. A simple microscope with a focal length of \( 5 \, \text{cm} \) forms an image at \( 25 \, \text{cm} \). What is the magnification?