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Lenses. Eyeglasses. Peep-hole. Watch. Camera. Security mirror. Magnifying glasses. 1 In daily life, you may have come across lenses that are thicker or thinner in the middle. Which of the following items do you think may contain lenses ?. . . . . .
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Eyeglasses. Peep-hole. Watch. Camera. Security mirror. Magnifying glasses. 1 In daily life, you may have come across lenses that are thicker or thinner in the middle. Which of the following items do you think may contain lenses?
2 When you see through lenses, do the objects appear the same as they are? No !
Introduction • Lenses are very useful. • e.g. in cameras, projectors, telescopes, microscopes and eyes • The most common lenses: • spherical lenses (i.e. the surface is spherical)
1 Convex and concave lenses Convex lens - thicker at the centre than at the edge.
1 Convex and concave lenses Concave lens - thinner at the centre than at the edge.
1 Convex and concave lenses a Converging or Diverging? convex lens concave lens (converging lens) (diverging lens)
1 Convex and concave lenses a Converging or Diverging? convex lens concave lens (converging lens) (diverging lens)
1 Convex and concave lenses a Converging lens bend the light inwards You may remember ‘converging’ as ‘concentrating’.
1 Convex and concave lenses a Diverging lens bend the light outwards You may remember ‘diverging’ as ‘dividing’.
1 Convex and concave lenses a Converging or diverging? Recall that light travels more slowlyin glass than in air. Light converges or diverges as some parts of the wavefronts lag behind. converges diverges
b Key features of lenses • Parallel rays are refracted inwards. principal focus • Refracted rays meet at a point called the principal focus F.
Centre of the lens is called the optical centre C. F optical centre
The distance of F from C is the focal length f of the lens. F C focal length
F' F F' F • A convex lens has 2 principal foci. • 1 on each side C principal axis • The line through the optical centre and 2 foci is called the principal axis.
Parallel rays are refracted outwards. principal focus • Refracted rays appear to spread from a point called the principal focus F.
Centre of the lens is called the optical centre C. F optical centre
The distance of F from C is the focal length f of the lens. F C focal length
F' F F' F • A concave lens has 2 principal foci. • 1 on each side C principal axis • The line through the optical centre and 2 foci is called the principal axis.
Q1 A light ray is incident on a… A light ray is incident on a convex lens. Which one represents the path of the light ray? X Y Z A Path X. B Path Y. C Path Z.
Q2 A light ray is incident on a… A light ray is incident on a concave lens. Which one represents the path of the light ray? X Y Z A Path X. B Path Y. C Path Z.
2 Image formation by a lens O screen When the light from a point of an object enters our eyes, we can see the point. I
2 Image formation by a lens I O a Real images Light rays converge to a point.
2 Image formation by a lens I O screen Image can be captured by a screen. Hence called ‘real’.
2 Image formation by a lens I O b Virtual images Light rays diverge from a point. No rays actually come from the image. Hence called ‘virtual’. convex lens
2 Image formation by a lens I O b Virtual images Light rays diverge from a point. concave lens
I O screen Since only convex lensesconverge light rays, real images can only be formed by convex lenses.
Light rays through lenses Video Video Video Simulation Simulation
3 Graphical construction of image formation a Construction rules In ray diagrams, we use symbols to represent lenses. convex lenses concave lenses
Construction rules for convex lenses Rule 1 A ray parallel to the principal axis... F C …is refracted through F.
Construction rules for convex lenses Rule 2 A ray passing through F'... F F' C …is refracted parallel to the principal axis.
Construction rules for convex lenses Rule 3 A ray passing through C... F F' C …travels straight on.
Construction rules for concave lenses Rule 1 A ray parallel to the principal axis... C F' F …is refracted so that it appears to come from F'.
Construction rules for concave lenses Rule 2 A ray directed towards F... C F' F …is refracted parallel to the principal axis.
Construction rules for concave lenses Rule 3 A ray directed towards C... C F' F …travels straight on.
3 Graphical construction of image formation b Drawing ray diagram Click the following to see how to draw ray diagrams. convex lens concave lens
Q1 Which of the following... A B F F’ F F’ C F F’ Which of the following incident rays is mentioned in the construction rules?
Q2 Which of the following light… A F’ F B C Which of the following light rays is correct ?
Q3 How to find the focus of the... Step 1: Draw a light ray A parallel to the principal axis from the tip of the arrow. O I
Q3 How to find the focus of the... Step 2: By the construction, the ray A appears to come from the ______ and the extended part ________ the tip of the image. focus passes O I
Q3 How to find the focus of the... Step 3: The point where the extended refracted light ray A cross the ______________ is the focus. principal axis O I F
F I C F' O 3 Graphical construction of image formation c Magnification magnification m height of image (image size) = height of object (object size)
u v F I C F' O 3 Graphical construction of image formation Also, image distance v m = magnification m = u object distance
4 Position and nature of image f Convex lens refracts the rays to form an inverted image on a plane at the principal focus. convex lens focal plane Such a plane is called the focal plane.
construction ray C F' F I We can find the position and nature of image by drawing ray diagram ∴ The 2 parallel rays also converge toI.
4 Position and nature of image I Images formed by a convex lens F 2F C 2F' F' at infinity object: image: at F , real, inverted and diminished
I O 2F F C 2F' F' object: beyond 2F ’ image: between F and 2F real, inverted and diminished
I O 2F F C 2F' F' object: at 2F ’ image: at 2F real, inverted and same size
