# I got marked incorrect for the following and need to verify with someone because I need to see why I am wrong please: Calculate the speed of laser light with a frequency of 2.857x10^14 Hz producing...

I got marked incorrect for the following and need to verify with someone because I need to see why I am wrong please:

Calculate the speed of laser light with a frequency of 2.857x10^14 Hz producing red light with a wavelength of 700x10^-9 as it travels through a glass.

I got 199,990,000 m/s.  What have I done wrong?

t-rashmi | College Teacher | eNotes Newbie

Posted on

The speed 199,990,000m/s is the speed of light in the glass. The question asks you to find out the speed of laser light, which means that you are supposed to find out the speed of the laser light in vacuum. The steps to find out the answer are given below.

GIVEN:

Frequency = 2.857x10^14Hz

Wavelength = 700x10^-9m

SOLUTION:

Velocity = Frequency`*`Wavelength

= (2.857x10^14Hz)`*`(700x10^-9m)

= 199,990,000m/s

This is the velocity of the laser light in glass.

We know,

Refractive Index = (Speed of light in vacuum)/(Speed of light in given medium)

Refractive Index of Glass is 1.5

Let the speed of laser light in vacuum be c.

`:. 1.5 = c/(199990000m/s)`

`:. c = 1.5 * 199990000m/s`

`:.c = 299985000m/s`

` `