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Finding the Focal Length of a Lens.

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Finding the Focal Length of a Lens

From Activity 23 (1)

The Into

Lenses have fascinated physicists ever since… well they have fascinated Mr. E. Allen so that’s more than enough.

Anyway, lenses can be used to modify images in a number of ways. They can be used to focus, blur, enlarge, reduce (which is still technically an enlargement with a factor less than one) and flip an image upside-down.

With all of these properties, there must be some rules governing how a lens works. In physics this means there are some mathematical rules, which can be shown through equations. For lenses, there is the lens equation, which states that when the image is focused:


u – Object to lens distance

v – Lens to image distance

ƒ – Focal length

The Lens Equation (2)

There is also another equation relating the focal length of a lens to the power of the lens:

image01.png and thus image02.png

P – Power of a lens in dioptres (D)

ƒ – Focal length in metres

If we know the power of a lens, we can work out the focal length and likewise, if we know the focal length we can work out the power. Once we have the focal length of a lens, we can work out the object-lens distance and/or the lens-image distance through the lens equation.

There is even more.

...read more.


  • The object-lens distance must be greater than the focal length of the lens; otherwise a magnification of the object occurs. This image can be projected but cannot be focused onto the screen (this image is known as a virtual image).
  • However, this effect became useful, because we used another lens to magnify the image on the screen to see whether or not the image (dots) were in focus. (See third bullet point)

After doing the pre-test, the problems encountered can be rectified in the experiment proper.

Now all that is left is to make some measurements of u and v when the image on the screen is focused, put them into the lens equation and see whether or not the focal length is 20 cm.

The Method

  1. Gather the required equipment: A lens, paper (to act as a screen), a stand to hold the screen, a double light source (plus power pack).
  2. Align the light source, lens and screen. Then focus the image from the light source onto the screen for the red light. To ensure that the image is focused, use another lens to magnify the image that is on the screen.
  3. Measure the distances for u and v and record the results.
  4. Repeat steps 2 & 3 for the green light.

The Safety (Issues)

...read more.


My results show that the focal length decreases as the frequency of light increases (i.e. focal length is inversely proportional to the frequency of light used). They also show that the focal length of a 5D lens, when measured with red and green light, is more that 200 mm. This leads me to conclude that when the manufacturers measured the power of the lens, they must have used a light with a higher frequency than that of green light. Since the power of the lens is 5D, the focal length should be 200 mm, and the only way for the focal length to be 200 mm is to use a higher frequency light source.

The Evaluation

As mentioned earlier, having a shaped light source would have made the experiment a lot more accurate than they currently stand.

The Reference

(1)         Salters Horners AS Advanced Physics

         Unit 1 (Physics at Work, Rest and Play)

         Section 3 (The Sound of Music)

         Pg. № 159/160

         Activity 23

(2)         Salters Horners AS Advanced Physics

         Unit 1 (Physics at Work, Rest and Play)

         Section 3 (The Sound of Music)

         Pg. № 159

(3)         Salters Horners AS Advanced Physics

         Unit 1 (Physics at Work, Rest and Play)

         Section 3 (The Sound of Music)

         Pg. № 161

(4)         “The refractive index of a material varies with frequency (except in

         vacuum, where all frequencies travel at the same speed, c).


         (about 11 paragraphs in. The above can be found on the paragraph

          above the table)


...read more.

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