*Question*

Three quantities used to describe a light wave are

frequency

wavelength

speed.

Which quantities increase when the light wave passes from water to air?

A I and II only

B I and III only

C II and III only

D I, II and III

**Answer/Explanation**

Ans: C

**Hence as velocity increases Wavelength will also increase**

*Question*

What are the changes in the speed and in the wavelength of monochromatic light when the light passes from water to air?

**Answer/Explanation**

### Markscheme

A

**Hence as velocity increases Wavelength will also increase**

*Question*

A pair of slits in a double slit experiment are illuminated with monochromatic light of wavelength 480 nm. The slits are separated by 1.0 mm. What is the separation of the fringes when observed at a distance of 2.0 m from the slits?

A. 2.4 × 10^{–4} mm

B. 9.6 × 10^{–4} mm

C. 2.4 × 10^{–1} mm

D. 9.6 × 10^{–1} mm

**Answer/Explanation**

### Markscheme

D

\(\beta =\frac{\lambda D}{d}\)

\(=\frac{480\times 10^{-9} \times 2}{10^{-3}}\)

\(=9.6\times 10^{-4}=9.6\times 10^{-1} mm\)

*Question*

The diagram shows an interference pattern produced by two sources that oscillate on the surface of a liquid.

Which of the distances shown in the diagram corresponds to **one **fringe width of the interference pattern?

**Answer/Explanation**

### Markscheme

C

Fringe width is **the distance between two consecutive bright spots (maximas, where constructive interference take place) or two consecutive dark spots (minimas, where destructive interference take place)**.

*Question*

The refractive index for light travelling from medium X to medium Y is \(\frac{4}{3}\). The refractive index for light travelling from medium Y to medium Z is \(\frac{3}{5}\). What is the refractive index for light travelling from medium X to medium Z?

A. \(\frac{4}{5}\)

B. \(\frac{15}{12}\)

C. \(\frac{5}{4}\)

D. \(\frac{29}{15}\)

**Answer/Explanation**

### Markscheme

A

\(^X\mu _Y =\frac{\mu_Y}{\mu_X}=\frac{4}{3}\)

\(^Y\mu _Z =\frac{\mu_Z}{\mu_Y}=\frac{3}{5}\)

\(^X\mu _Z =\frac{\mu_Z}{\mu_X}=\frac{\mu_Y}{\mu_X} \times \frac{\mu_Z}{\mu_Y} =\frac{4}{3} \times \frac{3}{5} =\frac{4}{5}\)

*Question*

When a sound wave travels from a region of hot air to a region of cold air, it refracts as shown.

What changes occur in the frequency and wavelength of the sound as it passes from the hot air to the cold air?

**Answer/Explanation**

### Markscheme

B

now \( i> r\) hence \(\mu_{cold}>\mu_{hot}\)

hence

\(v_{cold} <v_{hot}\) or \(\lambda_{cold}<\lambda_{hot}\) as \(v=f\lambda\)

Velocity decreases

Frequency does not depend on medium