The radius of the convex surface of planoconvex lens is 20 cm and the refractive index of the material of the lens is 1.5. The focal length is

• 30cm

• 50 cm

• 30 cm

• 40 cm

A short linear object of length b lies along the axis of a concave mirror of focal length ' f ' at a distance 'u' from the pole of the mirror, what is the size of image?

• $\left(\frac{\mathrm{f}}{\mathrm{u}-\mathrm{f}}\right) \mathrm{b}$

• ${\left(\frac{\mathrm{f}}{\mathrm{u}-\mathrm{f}}\right)}^2 \mathrm{b}$

• $\left(\frac{\mathrm{f}}{\mathrm{u}-\mathrm{f}}\right) {\mathrm{b}}^2$

• $\left(\frac{\mathrm{f}}{\mathrm{u}-\mathrm{f}}\right)$

The refractive index of the material of the prism and liquid are 1.56 and 1.32 respectively. What will be the value of 0 for the following refraction?

• $\sin \mathrm{\theta }\ge \frac{13}{11}$

• $\sin \mathrm{\theta } \ge \frac{11}{13}$

• $\sin \mathrm{\theta } \ge \frac{\sqrt{3}}{2}$

• $\sin \mathrm{\theta }\ge \frac{1}{\sqrt{2}}$

What should be the minimum value of refractive index of the material of the prism for the reflections to take place as shown in the figure? 

• 1.7

• 1.4

• 1.2

• 2.7

A person who can see things most clearly at a distance of 10 cm, requires spectacles to enable to see clearly things at a distance of 30 cm. What should be the focal length of the spectacles?

• 15 cm (Concave)

• 15 cm (convex)

• 10 cm

• 0

1f the focal length of a lens of a camera is 5f and that of another is 2.5f, what is the time of exposure for the second, if for the first one is $\frac{1}{200} \mathrm{s}$ ? (where f is focal length / unit aperture)

• $\frac{1}{200}$

• $\frac{1}{800}$

• $\frac{1}{6400}$

• $\frac{1}{3200}$

217.

If a convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined together, what will be their resultant power?

• +0.65 D

• -0.65 D

• +0.75 D

• -0.75 D

A plane glass slab is kept over various colour letters, the letter which appears least raised is :

• red

• violet

• green

• blue

A convex lens forms a full image of the object on a screen. If half of the lens is covered with an opaque object, then :

• the image disappears

• half of the image is seen

• full image is seen and of same intensity

• full image is seen and of decreased intensity

The time taken by light to pass through 4 mm thick glass slab of refractive index 1.5 will be : (velocity of light in air= 3 x 10⁸ m/s)

• 8 × 10^-11 s

• 2 × 10^-11 s

• 8 × 10^-8 s

• 2 × 10^-8 s