The wavelength of green light in an acrylic block is $3.60\times 10^{-7}m$.

The speed of green light in the acrylic block is $1.92\times 10^{8}m/s$.

Calculate the frequency of this green light in the acrylic block.

Use the equation: $wave\; speed=frequency\times wavelength$

Give your answer to 2 significant figures.

A sign has a green background with a white letter P on it.

What is the colour of the background and the letter P when viewed under pure red light?

• Colour of background: $\dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots$
• Colour of letter P: $\dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots$

Under green light, the letter P is difficult to read. Explain why.