NEET Physics Notes Magnetostatics EMI and AC, EM waves-Ray Optics-Lens
A lens is part of a transparent refracting medium bound by two surfaces, with atleast one of the two surfaces being a curved one. The curved surface may be spherical or cylindrical. The lens formula is given by
For a thin object of height h placed perpendicular to the principal axis at a distance u, if the height of image formed is h’, then lateral or transverse magnification m is given by
For a small sized object placed linearly along the principal axis, its axial or longitudinal magnification is given by
Let an object be situated at a distance x1 from the first principal focus and its image is formed at a distance x2 from the second principal focus, then x1x2 = f2
Power of Lens
The power of a lens is mathematically given by the reciprocal of its focal length, i.e. power
SI unit of power is dioptre (D). Power of a converging lens is positive and that of a diverging lens is negative.
Lens Maker’s Formula
For a lens having surfaces with radii of curvature R1 and R2 respectively and its focal length is given by
where,refractive index of the lens material w.r.t. the surroundings.
Cutting Of a Lens
If a symmetrical convex lens of focal length is cut into two parts along its optic axis, then focal length of each part (a plano-convex lens) is 2/. However, if the two parts are joined as shown in the figure, the focal length of the*combination is again .
If a symmetrical convex lens of focal length is cut into two parts along the principal axis, then the focal length of each part remains unchanged, as (b). If these two parts are joined with the curved ends on one side, the focal length of the combination is But on joining the two parts in opposite sense, the net focal length
The equivalent focal length of co-axial Combination of two lenses is given by
If a number of lenses are in contact, then If two thin lenses of focal lengths f1and f2 are in contact, then their equivalent focal lengthIn terms of power,
Total Internal Reflection (TIR)
When a ray of light goes from a denser to a rarer medium, it bends away from the normal. For a certain angle of incidence ic, the angle of refraction in rarer medium becomes 90°. The angle ic is called the critical Angle.
For the angle of incidence greater than the critical angle (i > ic) in the denser medium, the light ray is totally internally reflected back into the denser medium itself.
Conditions for Total Internal Reflection
- The light ray should travel from the denser medium towards the rarer medium.
- The angle of incidence should be the greater than the critical angle.
Deviation by a Prism
A prism is a homogeneous, transparent medium bounded by two plane surfaces inclined at an angle A with each other. These surfaces are called as refracting surfaces and the angle between them is called angle of prism A. Deviation produced by a prism is
For grazing incidence i = 90° and grazing emergence i’ = 90° For minimum deviation
In case of minimum deviation, ray is passing through prism symmetrically.
Dispersion by a Prism
Dispersion of light is the phenomenon of splitting of white light into its constituent colours on passing light through a prism. This is because different colours have different wavelength, and hence different refractive indices.
Refraction Through a Prism
A ray of light suffers two refractions at the two surfaces on passing through a prism.
Angle of deviation through a prism . where, i is the angle of incidence, e is the angle of emergence and A is the angle of prism.