What factors determine the focal length of a lens?

In summary, the focal length of a lens is determined by the curvature of one side, with the other side being flat. The size or thickness of the lens does not affect the focal length, but the diameter can impact the amount of light or angles that the lens can receive. Additionally, the word "lense" is incorrect, as "lens" is both the singular and plural form.
  • #1
sskk221
10
1
Let's say that there are three lenses A (all of which are semicircles: flat on one side, curved on the other). One lense is a short and curved D, another lense is a very elongated and narrow D (longer than the first lense), and the third lense is as long as the second lense but has a sharper curvature. How would you rank the focal lengths of these lenses? In other words, how is focal length of a lense related to its shape? More curved = shorter focal length? Longer lens = longer focal length?
 
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  • #2
The more steeply curved the shorter the focal length = the greater the bending effect on the light. A completely flat piece of glass has an infinite focal length and no bending effect
 
  • #3
I have a similar question, i knew about the curvature, but how does the SIZE of the lens affect the focal length? do smaller (shorter) lenses have shorter focal points than longer lenses?
 
  • #4
No - only the curvature affects the focal length
 
  • #5
Thanks! So, let's continue with the D-shaped lenses, if you have a short lense and a tall lense, and the width of the lenses (from the center of flat edge to the center of the curved edge) are equal, the short lens would have a smaller focal length since its relative curvature is higher than the longer lense, which doesn't have as steep of a curve (despite having equal width)?
 
  • #6
The focal length of a lens is set by the 'lens maker equation'
1/f = n(1/r1 - 1/r2)

D shaped (we call them plano-convex) only depend on the curvature of of one side since the other side is flat (ie r=infinite and 1/r=0)

The thickness of the lens doesn't matter (as far as focal length is concerned)

The diameter of the lens does matter for the amount of light the lens can receive (important in the first element of binoculars or a telescope) or for the range of angles it can receive light from (important in the elements at the back of a camera lens)

ps. Technically we call an individual 'lens' (ie one bit of glass) an element. A system of several elements working together - like in your camera is called a 'lens'.But when you only have a single element - like in a magnifying glass - this is also called a lens.
There is no such word 'lense' people think this because in english the plural is lenses
 

1. What is the definition of focal length for a thin lens?

The focal length of a thin lens is the distance between the lens and its focal point when parallel rays of light are focused to a point. It is a measure of the lens's ability to bend light and determine the size of the image that is formed.

2. How is the focal length of a thin lens calculated?

The focal length of a thin lens can be calculated using the lens maker's formula: f = (n-1)(1/R1 - 1/R2), where f is the focal length, n is the refractive index of the lens material, and R1 and R2 are the radii of curvature of the lens surfaces.

3. Can the focal length of a thin lens be changed?

Yes, the focal length of a thin lens can be changed by altering the curvature of the lens surfaces or by changing the refractive index of the lens material. This can be done by using different lens materials, adding or removing layers of material, or by using external devices such as adjustable lenses or mirrors.

4. How does the focal length affect the image formed by a thin lens?

The focal length of a thin lens determines the size and location of the image formed. A longer focal length will result in a larger image, while a shorter focal length will result in a smaller image. The distance between the lens and the image also depends on the focal length - a shorter focal length will result in a shorter distance between the lens and the image.

5. What is the difference between a positive and negative focal length in a thin lens?

A positive focal length means that the lens is converging, meaning parallel rays of light are brought together to a point after passing through the lens. A negative focal length means that the lens is diverging, meaning parallel rays of light are spread apart after passing through the lens. This affects the type of image formed - a converging lens will produce a real image, while a diverging lens will produce a virtual image.

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