1© Manhattan Press (H.K.) Ltd. Final image at infinity Eye-ring Eye-ring 12.6 Refracting telescope.

$: 1© Manhattan Press (H.K.) Ltd. Final image at infinity Eye-ring Eye-ring 12.6 Refracting telescope.$
© Manhattan Press (H.K.) Ltd. 1

• Final image at infinityFinal image at infinity• • Eye-ringEye-ring

12.6 Refracting 12.6 Refracting telescopetelescope


12.6 Refracting telescope (SB p. 233)

Refracting telescope

Refracting telescope- consists of two converging lenses(objective & eyepiece)



Final image at infinity

Go to

More to Know 14More to Know 14



Final image at infinity

o

ef/hf/h

M

e

off

M

Increase M by:1. use objective of longer focal length2. use eyepiece of shorter focal length

Go to


Go to

Example 13Example 13



Eye-ring

In general, the eye-ring defines the smallest region which all refracted lights by both lenses have passed through.



Eye-ring

Observer views through eye-ring- receives maximum amount of light

Go to


eoe

eeo111

111

111

fffd

fdff

fvu

d – distance of eye-ring from eyepiece



12.1 12.1 Reflection and refractionReflection and refraction

1. The Laws of Reflection:(a) The incident ray, the normal to the surface and the reflected ray are all lie in one plane.(b) The angle of reflection is equal to the angle of incidence.




2. The properties of the image formed by a plane mirror:(a) virtual (an image cannot be formed on a screen)(b) erect(c) same size as the object(d) laterally inverted(e) the image distance is equal to the object distance




3. When light travels across the interface of two media, refraction occurs because of the change in light speed.

4. The Laws of Refraction:(a) The incident ray, the normal and the refracted ray at the point of incidence are all lie in one plane.




4. (b) At the interface between any two different media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for any particular wavelength of the ray. This is named as Snell’s Law.

n1 sinθ1 = n2 sinθ2




5. Refractive index of a medium =

where real depth is the separation between the interface and the object and apparent depth is the separation between the interface and the image.

6. Refractive index of a prism =

where A is the refracting angle of the prism and Dmin is the minimum deviation.

depthApparent depth Real

2sin

2sin min

A

DA




7. Refraction by rectangular glass blockLateral displacement (d) =

where θ1 is the angle of incidence, θ2 is the angle of refraction and a is the width of the rectangular glass block.

2

21cos

sin

a




8. When the angle of incidence > c (critical angle of a medium), total internal reflection occurs. The light rays must travel from a medium with high refractive index to one with low refractive index.

nc 1sin



12.2 12.2 Converging and diverging lensesConverging and diverging lenses

9. A converging (convex) lens causes light rays to converge while a diverging (concave) lens causes light rays to diverge.

10. The parts of lenses:(a) optical centre (O)(b) principal axis(c) principal focus (F)(d) focal length (f)(e) focal plane




11. There are three special incident light rays for lenses:(a) a ray passes through the optical centre O(b) a ray is parallel to the principal axis(c) a ray passes through F (for converging lens) or is directed towards F (for diverging lens)




12. The nature of the images formed by a converging lens: where u is the object distance and v is the image distance.

Object position

Image position

Nature of image

at infinity at F real, inverted, diminished

u > 2f f < v < 2f real, inverted, diminished

u = 2f v = u = 2f real, inverted, same size as the object




12. The nature of the images formed by a converging lens:

Object position

Image position

Nature of image

f < u < 2f v > u real, inverted, magnified

u = f at infinity cannot be determined

u < f behind object virtual, erect, magnified




13. The nature of the images formed by a diverging lens:(a) virtual(b) erect(c) diminished(d) v < f




14. The lens formula relates the object distance (u), the image distance (v) and the focal length (f ).

(a) If the object and the image are real, then the values of u and v are positive.(b) If the object and the image are virtual, then the values of u and v are negative.(c) The value of f of a converging lens is positive while the value of f of a diverging lens is negative.

fvu111




15. The linear magnification (m) is:

16. When two thin lenses of focal lengths f1

and f2 are in contact,

where f is the combined focal length.

)( distanceObject )( distance Image

)(height Object )(height Image

o

iuv

hh

m

21

111fff



12.3 12.3 Properties of visionProperties of vision

17. The far point is the point furthest from the eye where an object can be seen clearly by the eye without straining it.

18. The near point is the point of the least distance from the eye such that an object can be seen clearly by the eye without straining it.

19. The least distance of distinct vision is the distance of the near point from a normal eye.



12.3 12.3 Properties of visionProperties of vision

20. A short-sighted person cannot see distant objects clearly and it can be corrected by a diverging lens.

21. A long-sighted person can only see distant objects clearly and it can be corrected by a converging lens.

22. Visual angle is the angle subtended at the eye by the object.



12.4 12.4 Magnifying glassMagnifying glass23. A magnifying glass is a converging lens.

24. The angular magnification (M) is the power of the magnification of the magnified image by a magnifying glass.

where D is the distance between the near point and the eye (least distance of distinct vision) and f is the focal length of the magnifying glass.

1

)(object by the eye unaided at the subtended angle Visual)( image by the eye at the subtended angle Visual

fD

M



12.5 12.5 MicroscopeMicroscope

25. Two converging lenses (objective and eyepiece) are used in a microscope.

26. An intermediate image is formed by the objective within the focus of the eyepiece. Therefore, the eyepiece acts as a magnifying glass to form a magnified image at the least distance of distinct vision (near point).



12.5 12.5 MicroscopeMicroscope

27. The angular magnification (M) of a compound microscope is:M = Linear magnification of eyepiece (me) x Linear magnification of objective (mo)



12.6 12.6 Refracting telescopeRefracting telescope

28. A refracting telescope consists of two converging lenses (objective and eyepiece) as the microscope.

29. An intermediate image is formed by the objective on the focal plane of the eyepiece. Therefore, the eyepiece acts as a magnifying glass to form a final image at infinity.



12.6 12.6 Refracting telescopeRefracting telescope

30. The angular magnification (M) of a telescope is:

where fo is the focal length of the objective and fe is the focal length of the eyepiece.

31. The eye-ring of a telescope or a microscope defines the smallest region which all refracted lights by the objective and eyepiece have passed through.

e

off

M


End


Normal adjustment of telescope

A telescope in normal adjustment forms the final image at the far point of the user (at infinity) because it is always used to view distant objects.

Return to

TextText



Length of telescope tube

The length of the telescope tube must be greater than the sum of fo and fe.

Return to

TextText



Large aperture of telescope

Telescopes always have a large aperture because they can minimize the effect of diffraction and collect more light from the distant object.

Return to

TextText



Q: Q: A telescope whose objective has a focal length of 60 cm and its eyepiece has a focal length of 1.5 cm. Calculate(a) the separation between the lenses forming an image at infinity, and(b) the angular magnification of the telescope.

Solution



Solution:Solution:(a) The lens separation is estimated as: = fo + fe = 60 + 1.5 = 61.5 cm

4051

60

:)(ion magnificatangular The (b)

e

o .f

fM

Return to

TextText


1© Manhattan Press (H.K.) Ltd. Final image at infinity Eye-ring Eye-ring 12.6 Refracting telescope.

Documents

Transcript of 1© Manhattan Press (H.K.) Ltd. Final image at infinity Eye-ring Eye-ring 12.6 Refracting telescope.