21.

The speed of light in vacuum is equal to

• µ₀ε₀

• µ₀²ε₀²

• $\frac{1}{\sqrt{{\mathrm{\mu }}_0{\mathrm{\epsilon }}_0}}$

• $\sqrt{{\mathrm{\mu }}_0{\mathrm{\epsilon }}_0}$

When an object is viewed with a light of wavelength 6000 $\stackrel{\circ }{\mathrm{A}}$ under a microscope, its resolving power is 10⁴. The resolving power of the microscope when the same object is viewed with a light of wavelength 4000 $\stackrel{\circ }{\mathrm{A}}$, is

• 10⁴

• 2 × 10⁴

• 1.5 × 10⁴

• 3 × 10⁴

Secondary rainbow in the atmosphere is

• the result of polarization and dispersion of light

• brighter than the primary rainbow

• formed due to two reflections in the rain drop

• formed with red colour on the top

If the radius of curvature of the curved surface of a plano -convex lens is 50 cm, its focal length is (µ =1.5)

• 0.5 m

• 0.75 m

• 1 m

• 0.25 m

The magnification of an image by a convex lens is positive only when the object is placed

• at its focus F

• between F and 2F

• at 2F

• between F and optical centre

A ray of light is incident normally on one refracting surface of an equilateral prism. If the refractive index of the material of the prism is 1.5, then

• the emergent ray is deviated by 30°

• the emergent ray is deviated by 60°

• the emergent ray just graces the second reflecting surface

• the ray undergoes total internal reflection at second refracting surface

An aperture of size a is illuminated by a parallel beam of light of wavelength λ. The distance at which ray optics has a good approximation is

• $\frac{{\mathrm{a}}^2}{\mathrm{\lambda }}$

• $\frac{\mathrm{\lambda }}{{\mathrm{a}}^2}$

• $\frac{\mathrm{\lambda }}{\mathrm{a}}$

• $\frac{{\mathrm{\lambda }}^2}{\mathrm{a}}$

Two plane wavefronts of light, one incident on a thin convex lens and another on the refracting face of a thin prism. After refraction at them, the emerging wavefronts respectively become

• plane wavefront and plane wavefront

• plane wavefront and spherical wavefront

• spherical wavefront and plane wavefront

• spherical wavefront and spherical wavefront

Identify the wrong sign convention.

• The magnification for virtual image formed by a convex lens is positive.

• The magnification for real image formed by a convex lens is negative

• The magnification for virtual image formed by a concave lens is negative

• The distances measured in the direction of incident light is positive

The apparent flattening of the sun at sunset and sunrise is due to

• refraction

• diffraction

• total internal reflection

• interference