The wavelength of blue light in a diamond crystal is $1.91\times 10^{-7}m$.

The speed of blue light in the diamond crystal is $1.24\times 10^{8}m/s$.

Calculate the frequency of this blue light in the diamond crystal.

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

Give your answer to 2 significant figures.

A warning sign has a blue background with a white letter S on it.

What is the colour of the background and the letter S when viewed under pure yellow 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 S: $\dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots$

Under blue light, the letter S is difficult to read. Explain why.