Optics & Light — Part 2: 14 MCQs with Explanations

Paschen series (transitions to n=3) has wavelengths > 820 nm, in infrared region.

Davisson and Germer (1927) observed electron diffraction by nickel crystal, confirming de Broglie hypothesis.

Balmer-α line: 1/λ = R(1/2² - 1/3²) = 5R/36 ⇒ λ = 36/(5R) ≈ 656 nm (red light).

Power of a lens P = 1 / f (in meters). A convex lens has a positive focal length. Here, f = +20 cm = +0.2 m. Therefore, P = 1 / 0.2 = +5 diopters (D).

The intensity of light is related to the number of photons incident per second per unit area. A higher intensity means more photons, which can eject more photoelectrons, increasing the photocurrent.

X-rays have wavelengths in the range of about 10⁻⁸ m to 10⁻¹¹ m. 10⁻¹⁰ m (0.1 nm) falls within the X-ray region of the electromagnetic spectrum.

Diffraction is the slight bending of light as it passes around the edge of an object or through a narrow slit. The amount of bending depends on the size of the obstacle relative to the wavelength of light.

A plane mirror always forms an image that is virtual (cannot be projected on a screen), erect (upright), and laterally inverted (left appears as right). The image is as far behind the mirror as the object is in front.

Myopia (nearsightedness) occurs when the eyeball is too long or the eye lens is too curved. This causes the image of a distant object to be formed in front of the retina, making it appear blurry.

A convex mirror always forms an image that is virtual (behind the mirror), erect, and diminished (smaller than the object). Its focal length is considered negative by the sign convention.

A concave lens (diverging lens) always forms an image that is virtual (on the same side as the object), erect, and diminished, regardless of the object's position.

The critical angle θ is given by sin θ = n₂/n₁ = v₁/v₂, where v₁ is the speed in medium A and v₂ is the speed in medium B. Therefore, v₂ = v₁ / sin θ = v / sin θ.

Power P = 1/f (in meters). A positive power indicates a convex lens. So, f = 1/P = 1/2 = 0.5 m = 50 cm.

When light enters a denser medium, it slows down and bends towards the normal. Therefore, the angle of refraction (r) is less than the angle of incidence (i).