Test: Reflection of Light by Spherical Mirrors & Refraction (February 17) - NEET MCQ

for concave mirror u=−7.5 ; f=−5
(1/v)+1/u=1/f
1/v-2/15=-1/5
i.e, image formed at pole of convex mirror for convex mirror, u=0;f=+5
(1/v)+1/u=1/f
(1/v)+1/0=1/5
V=0
So the final image is formed on a pole of a convex mirror.

Only Convex Mirror can form an image which is upright and reduced in size.

A real image is defined as one that is formed when rays of light are directed in a fixed point.  A real image can be projected or seen on a screen. The best example of a real image is the one formed on a cinema screen.
A virtual image is defined as the opposite of a real image, therefore an image that cannot be obtained on a screen is referred to as a virtual image. The explanation for this is the fact that the rays of light that form a virtual image never converge therefore a virtual image can never be projected onto a screen. The best example of a virtual image is your reflection in the mirror.
Real images are produced by intersecting rays while virtual images are produced by diverging rays.
Real images can be projected on a screen while virtual ones cannot.
Real images are formed by two opposite lenses, concave and convex.
Virtual images are always upright while real images are always inverted.
 

Focal length in air = 20 cm
Refractive index of air-water n₁= 1.33
Refractive index of air - glass n₂= 1.5
For focal length in air,
Using formula of lens
1/f_air={(n₂/n₁)-1}(1/R₁)-(1/R₂)
Put the value into the formula
1/20={(1.5/1)-1}{(1/R₁)-(1/R₂)}
1/20=0.5{(1/R₁)-(1/R₂)}…1
We need to calculate the focal length in water
Using formula of lens
1/f_water={(1.5/1.33)-1}{(1/R₁)-(1/R₂)}
1/f_water=0.128{(1/R₁)-(1/R₂)}….2
f_water/20=0.5/0.128
f_water=78.125cm

M=−ϑ /u ​=−m/1​
ϑ=−mu
(1/ϑ​)−(1/u)​=1/f
1/mu​+1/u​=−1/f​
(1+m)​/xu=−1/f​
u= (1+m)​f/(m)

Consider refraction of light from infinity at first surface,
(μ2/v)​​−( μ1/u​)​=(μ2​−μ1/R)​​
(1.5​/ v1​)-(1.75/−∞)​=1.5−1.75​/−R
Consider refraction of this image from second surface,
​(1.75​/ v2)−(​1.5/v1)​=(1.75−1.5​)/R
Hence, v2​=3.5R
This is the image of light coming from infinity, therefore the focal length.
It is converging since focal length is positive.
 

A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century, by Isaac Newton, as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives.
Astronomical (reflecting) telescopes. In a reflecting telescope, instead of a convex objective lens, a concave mirror is used to collect parallel rays from the object and form an image at the focal point. Then the convex eyepiece lens is used to magnify this image for the viewer.