The focal length of a lens is inversely proportional to the quantity ( n ? 1), w
ID: 2245496 • Letter: T
Question
The focal length of a lens is inversely proportional to the quantity (n ? 1), where n is the index of refraction of the lens material. The value of n, however, depends on the wavelength of the light that passes through the lens. For example, one type of flint glass has an index of refraction of nr = 1.572 for red light and nv = 1.605 for violet light. Now, suppose a white object is placed 37.70 cm in front of a lens made from this type of glass. If the red light reflected from this object produces a sharp image 63.40 cm from the lens, where will the violet image be found?
cm from the lens
Explanation / Answer
we know for lens 1/f=1/v-1/u [both u &v are -ve]
f=93 cm=f(red)
now f(red)/f(voilet)=nv-1/(nr-1)
f(voilet)=87.93 cm
therefore 1/v=1/f+1/u [u will remain same]
v= -66 cm