Instructions • To run the slideshow: – Click: view – full screen mode, or press Ctrl +L. – Left click advances one slide, right click returns to previ...
Instructions • To run the slideshow: – Click: view – full screen mode, or press Ctrl +L. – Left click advances one slide, right click returns to previous slide. – To exit the slideshow press the Esc key.
Optical Bench- Convex Lenses KS4 Suitable for OCR A Gateway Science
On a copy of the diagram below 1.
Complete the path of the ray of light. (Remember to draw the normal lines.) Air
2.
State the name of the effect.
Glass
Light • Remember light will slow down as it travels from air to glass because glass is more optically dense. Air
Glass
• The light bends towards the normal on the way in and away from the normal on the way out. This is known as REFRACTION
Lenses • Simple lenses are curved pieces of glass or plastic. • They come in two basic forms:
Convex
Concave
• A convex lens is thicker in the middle than at the edges.
• A concave lens is thicker at the edges than in the middle.
Refraction • Using your knowledge of refraction, complete the three rays of light incident on a convex lens. (It is important here to accurately draw your normal lines.)
Refraction • As before, light entering the glass bends towards the normal and the light leaving the glass bends away from the normal.
• This causes the rays of light to be brought together. • For this reason convex lenses are often called CONVERGING lenses.
Drawing lenses • While light passing through a lens is refracted upon entering the glass and leaving the glass it is often easier to draw the refraction occurring at the centre of the lens.
Focal length • The point at which the parallel rays of light cross is called the focal point or focus. • The distance from the centre of the lens to the focal point is called the focal length. Focal point
Principal Axis Focal Length
• Add these labels to your diagram
Optical Bench • An optical bench allows lenses to be easily aligned and measurements taken.
• Set up your optical bench with the rule pointing at a distant object. • Arrange the screen and a lens holder on the rule so that light from the distant object can pass through the lens and strike the screen. • Put the one of the lenses in the holder and place it close to the screen. • Slowly move the lens away from the screen until a sharp image is formed on the screen.
Explanation • Light travelling parallel to the principal axis will be refracted through the focal point. • Light that passes through the centre of the lens will be undeviated. • We can then determine the position of an image by constructing a ray diagram. Object
Image
Distant Objects • The further the object is from the lens the closer the image gets to the focal length of the lens.
Object
Image
Determining focal length If you have focused a distant object onto your screen the distance between the centre of the lens and the screen will be an estimation of the focal length.
Using one of the lenses from your set: • Using a rule estimate the thickness of the lens at its widest point. • Place the lens in a holder.
• Slowly move the lens away from the screen until a sharp image is formed. • The distance from the lens to the screen is an approximation of the focal length of the lens. • Record your results in the table provided. • Repeat for the remaining lenses.
Results Thickness of lens (mm)
Estimated focal length f (mm)
15
60
9
110
5
200
Power • For convex lenses of the same material the more curved the surface of the lens is, the shorter the focal length. • We say that the shorter the focal length the more powerful the lens is. • We can calculate power using the following equation: 𝑝𝑜𝑤𝑒𝑟 =
1 𝑓𝑜𝑐𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ
(m-1 or dioptres)
(Take care with units, focal length should be measured in metres)
• Calculate the power of your three lenses and add them to your table. Which lens is the most powerful?
Results Thickness of lens (mm)
Estimated focal length f (mm)
Power (Dioptres)
15
60
16.6
9
110
9.1
5
200
5.0
Astronomical objects • Light from distant astronomical sources reaches Earth as effectively parallel rays. Focal Plane Image
• For an object off the principal axis an image is formed in the focal plane.
Telescopes • A telescope forms an image of a distant object using a convex lens as in the previous diagram. • In order to view that image we need a second lens to look through which magnifies the image. Distant Object
Magnified Image
Telescopes • The first lens is called the objective and the second lens the eyepiece. • To build a telescope the eyepiece must have a shorter focal length than the objective lens. Distant Object
Magnified Image
Building a telescope • Using two lens holders, the 200mm lens and the 50mm lens from your set, arrange the apparatus on the metre rule so that you can observe a distant scene. • Start with the two lenses close together and look through the eyepiece. Slowly move the objective lens away from you until a sharp image is visible. (Keeping your eye close to the eyepiece yields the best results.)
• Calculate the distance between the centre of the eyepiece and the centre of the objective lens using the markings on the metre rule. What do you notice?
Magnification • In a simple optical telescope: • The objective lens collects the light from a distant object and forms an image. • The eyepiece magnifies the image so that we can see it. • The total magnification of the telescope is given by: 𝑓𝑜𝑐𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑜𝑏𝑗𝑒𝑐𝑡𝑖𝑣𝑒 𝑙𝑒𝑛𝑠 𝑚𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 = 𝑓𝑜𝑐𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑒𝑦𝑒𝑝𝑖𝑒𝑐𝑒 𝑙𝑒𝑛𝑠
Notice magnification has no units as it is a ratio.
• Why do you think that the eyepiece must be more powerful than the objective lens in a telescope?
Experimenting with your telescope • Remember that the eyepiece must be more powerful for the telescope to work if not the image will be smaller than the object. • Substitute the following different lenses for the eyepiece and the objective lens. • Calculate the magnification for each arrangement. Test
Eyepiece Lens (mm)
Objective Lens (mm)
1
50
200
2
50
100
3
100
200
Magnification
• Which combination of lenses gives you the greatest magnification?
Results Test
Eyepiece Lens (mm)
Objective Lens (mm)
Magnification
1
50
200
4
2
50
100
2
3
100
200
2
• The greatest magnification is given with the 50mm lens as the eyepiece and the 200mm lens as the objective. • Draw a sketch of your telescope labelling the objective lens and the eyepiece. • State the function of the two lenses.
Telescopes • The objective lens forms an image of a distant object. • The eyepiece lens magnifies the image. Objective Lens Distant Object
Magnified Image
Eyepiece
Summary Questions • State what is meant by focal length. • Calculate the power of a convex lens with focal length 300mm. (Remember to convert units.) • A telescope is constructed which has an objective lens with a focal length of 400mm and an eyepiece of focal length 50mm. Calculate the magnification of the telescope.
Solutions • The focal length of a lens is the distance between the centre of the lens and the focal point. • 𝑝𝑜𝑤𝑒𝑟 =
