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Monochromatic light of a certain wavelength falls on a narrow slit. On a screen a certain distance away, the distance between the second minimum and the central maximum is 2cm. What would happen to the spacing between the two if the wavelength of the radiation were increased?

  • a) It would depend on whether the new wavelength is an odd or even multiple of the old.
  • b) It would remain unchanged
  • c) It would decrease.
  • d) it would increase.

D. It would increase.


Electromagnetic Spectrum

The following units are often used when quoting wavelengths:

  • 1 mm = 10− 3m,
  • 1 μ m = 10− 6 m,
  • 1 nm = 10− 9 m, and
  • 1 Å (angstrom) = 10− 10 m.

The full spectrum is broken up into many regions, which in descending order of wavelength are

  • radio (109 m− 1 m),
  • microwave (1 m− 1 mm),
  • infrared (1 mm− 700 nm),
  • visible light (700 nm− 400 nm),
  • ultraviolet (400 nm− 50 nm),
  • x-ray (50 nm− 10− 2 nm), and
  • gamma rays (less than 10− 2 nm).


electromagnetic waves


speed of light

 c=3 × 10m/s




  • used to describe an ideal radiator
  • refers to the fact that such an object is also an ideal absorber and would appear totally black if it were at a lower temperature than its surroundings


rectilinear propagation

When light travels through a single homogeneous medium, it travels in a straight line


 geometrical optics.

behavior of light at the boundary of a medium or interface between two media is described by this theory


law of reflection

θ 12

where θ 1 is the incident angle and θ 2 is the reflected angle, both measured from the normal


real image

light actually converges at the position of the image


virtual image

light only appears to be coming from the position of the image but does not actually converge there.

  • Because the light does not converge at all, plane mirrors always create virtual images


center of curvature (C)

a point on the optical axis located at a distance equal to the radius of curvature from the vertex of the mirror;

in other words, where the center of the mirrored sphere would be, were it a complete sphere.


radius of curvature (r) 

a measure of the radius of the circular arc which best approximates the curve at that point


Focal length (for all spherical mirrors)



Key Variable Distances for Geometrical Optics

The focal length (f) is the distance between the focal point (F) and the mirror (for all spherical mirrors,

, where the radius of curvature (r) is the distance between C and the mirror); the distance of the object from the mirror is o; the distance of the image from the mirror is i 


Mirror Equation

(1/o) + (1/i) = 1/f =1/f = 2/r


  • Plane mirrors can be thought of as infinitely large spherical mirrors.
  • As such, for a plane mirror, r = f = ∞ , and the equation becomes: (1/o) + (1/i) = 0
  • or :   i= (-o)


magnification (m)

 dimensionless value that is the ratio of the image's height to the object's height

m= -(i/o)

  • A negative magnification signifies an inverted image, while
  • a positive value means the image is upright.
  • If |m| < 1, ("absolute value of m is less than 1") the image is reduced; i
  • f |m| > 1, the image is enlarged; and
  • if |m| = 1, the image is the same size as the object.


Ray Diagrams for Concave Spherical Mirror at 3 Different Points

  • In Figure 10.6a, the object is placed between F and C, and the image produced is real, inverted and magnified.
  • In Figure 10.6b, the object is placed at F, and no image is formed because the reflected light rays are parallel to each other. In terms of the mirror equation, we say that the image distance i = ∞ here.
  •  Figure 10.6c, the object is placed between Fand the mirror, and the image produced is virtual, upright, and magnified.


In general, there are three important rays to draw..

What are they?

  1.  For a concave mirror, a ray that strikes the mirror parallel to the horizontal is reflected back through the focal point.
  2. A ray that passes through the focal point before reaching the mirror is reflected back parallel to the horizontal.
  3. A ray that strikes the mirror right where the normal intersects it gets reflected back with the same angle (measured from the normal).


Sign Chart for Single MIrrors



Positive (+)

Negative (-)


Object is in front of the mirror (R-side)

Object is behind mirror (V-side)


Image is in front of mirror (R-side)

Image is behind mirror (V-side)


Concave mirrors

Convex mirrors


Concave mirrors

Convex mirrors


Image is upright (erect)

Image is inverted


Snell's Law of Refraction

nsin θ1= nsin θ2


index of refraction of the medium. 


  • For air, n is essentially equal to 1 because v≈ c.
  • But for all other materials on MCAT:  v  1. 


General Bending concepts moving between indecies of different n-values:

  • when light enters a medium with a higher index of refraction (n2 > n1), it bends towards the normal so that θ 2 
  •  if the light travels into a medium where the index of refraction is smaller (n2  θ 1.


Total internal reflection

all the light incident on a boundary is reflected back into the original material, results for any angle of incidence greater than the critical angle, θ c 

sinθ =n1/n2



 two surfaces that affect the light path

Because light can be coming from either side of a lens, a lens has two focal points (one on each side of the lens) and the focal length can be measured in either direction from its center


basic formulas for finding image distance and magnification

(apply as those from spherical mirrors) 


 lensmaker's equation

For lenses where the thickness cannot be neglected, the focal length is related to the curvature of the lens surfaces and the index of refraction of the lens by: 

1/f =(n-1) [(1/r1)-(1/r2)]

where n is the index of refraction of the lens material, r1 is the radius of curvature of the first lens surface and r2 is the radius of curvature of the second lens surface.


Sign Chart for Single Lenses





Object on side of lens light is coming from (V side)

Object on side of lens is going to (R side)


Image on side of lens light is going to (R side)

Image on side of lens light is coming from (V side)


Converging lens

Diverging lens


Image erect

Image inverted


When on R side (convex surface as seen from side the light is coming from)

When on V side (concave surface as seen from side light is coming from)


power (P).


diopters, where f (the focal length) is in meters and is given by this equation:



P has the same sign as f and is, therefore, positive for a converging lens and negative for a diverging lens.


Lenses in contact

 series of lenses with negligible distances between them. These systems behave as a single lens with equivalent focal length given by


lenses not in contact,

image of one lens is used to make the object of another lens