Prepared by:
Ridwan Abrar
Department of EEE, BUET
4th year
© All rights reserved
Note: Mark schemes from many different years have been combined
to produce a better answer where necessary
Salient features of this note:
* It is always updated with questions from recent past papers
* It deals mainly with questions from past papers and leaves out
unnecessary details
* List of difficult questions from past papers which will help you
with last minute revision
* Detailed explanation of the difficult questions
Syllabus changes:
Excluded from Old syllabus: Doppler effect
Added to new syllabus: Band theory
** Note that the Pulse echo technique is still in the new syllabus. It frequently came along
with the Doppler effect in the past, so make sure you don’t leave that part out!
Waves
To remember the different regions of the electromagnetic spectra:
Rich Man in London use xtra gold
I wait on dance floor
Prepared by Ridwan Abrar
,The waves slow down in a denser medium like water or glass. Sometimes when a wave travels through a
medium, different wavelengths travel at different speed, this is called dispersion because the waves
spread out.
Displacement is the distance from the center of oscillation to the rope.
It is a vector so a trough is negative. The maximum displacement
occurs at a peak or a trough of the wave. It is called the amplitude of
the wave
Bigger amplitude waves carry more energy. The energy is directly
proportional to amplitude2
A ray is a very narrow band of waves and is drawn as a straight line in
the direction of movement of the wave.
Light is refracted because it slows down or speeds up.
One medium to another: relative refractive index; vacuum to another
medium: absolute refractive index
A refractive index of less than 1 means that light is speeding up, so it is
refracting away from the normal.
Frequency: Number of cycles/ oscillations/ waves per second/ per
unit time OR number of cycles/ oscillations/ waves passing a point
per second
Critical angle:
It is the smallest angle of incidence for which light travelling from a denser medium to less dense
medium, is not refracted or travels along the edge of the surface.
Or, It is the angle beyond which total internal reflection occurs
Conditions for Total internal reflection:
● When light travels from a denser to less dense medium
● the angle of incidence is greater than the critical angle, then total internal reflection takes place.
● No light is refracted, all light is reflected back into the denser medium.
Prepared by Ridwan Abrar
, The student increase the angle x in glass and finds that eventually the
light does not pass into the air. Explain this observation.
● As x increases, y increases
● OR At a certain angle or critical angle, y = 90o./ the light travels
along the boundary (do not allow reflects at 90o)
● For angles greater than the critical angle in glass, total internal
reflection occurs
Changes in density or concentration will change the refractive index
Two rays have same frequency/ come from same source/ are
coherent
Path difference (between the two reflected rays)
They superpose when they meet/ constructive and destructive
interference occurs
If they meet in phase, constructive interference/ bright fringe
If they meet in antiphase, destructive interference/ dark fringe
Coherent: Waves of constant phase relationship
Standing wave
No net transfer of energy or pattern of nodes/antinodes or points of
maximum displacement and zero displacement
Superposition along PQ [central maxima in Young’s double slit]
Constructive interference/ reinforcement/ waves of larger amplitude
Crests from S1 and S2 coincide
Amplitude is the sum of individual amplitudes
Explanation of refraction taking place
Change in speed/ density/ wavelength
How do sound waves travel through air?
Prepared by Ridwan Abrar
Ridwan Abrar
Department of EEE, BUET
4th year
© All rights reserved
Note: Mark schemes from many different years have been combined
to produce a better answer where necessary
Salient features of this note:
* It is always updated with questions from recent past papers
* It deals mainly with questions from past papers and leaves out
unnecessary details
* List of difficult questions from past papers which will help you
with last minute revision
* Detailed explanation of the difficult questions
Syllabus changes:
Excluded from Old syllabus: Doppler effect
Added to new syllabus: Band theory
** Note that the Pulse echo technique is still in the new syllabus. It frequently came along
with the Doppler effect in the past, so make sure you don’t leave that part out!
Waves
To remember the different regions of the electromagnetic spectra:
Rich Man in London use xtra gold
I wait on dance floor
Prepared by Ridwan Abrar
,The waves slow down in a denser medium like water or glass. Sometimes when a wave travels through a
medium, different wavelengths travel at different speed, this is called dispersion because the waves
spread out.
Displacement is the distance from the center of oscillation to the rope.
It is a vector so a trough is negative. The maximum displacement
occurs at a peak or a trough of the wave. It is called the amplitude of
the wave
Bigger amplitude waves carry more energy. The energy is directly
proportional to amplitude2
A ray is a very narrow band of waves and is drawn as a straight line in
the direction of movement of the wave.
Light is refracted because it slows down or speeds up.
One medium to another: relative refractive index; vacuum to another
medium: absolute refractive index
A refractive index of less than 1 means that light is speeding up, so it is
refracting away from the normal.
Frequency: Number of cycles/ oscillations/ waves per second/ per
unit time OR number of cycles/ oscillations/ waves passing a point
per second
Critical angle:
It is the smallest angle of incidence for which light travelling from a denser medium to less dense
medium, is not refracted or travels along the edge of the surface.
Or, It is the angle beyond which total internal reflection occurs
Conditions for Total internal reflection:
● When light travels from a denser to less dense medium
● the angle of incidence is greater than the critical angle, then total internal reflection takes place.
● No light is refracted, all light is reflected back into the denser medium.
Prepared by Ridwan Abrar
, The student increase the angle x in glass and finds that eventually the
light does not pass into the air. Explain this observation.
● As x increases, y increases
● OR At a certain angle or critical angle, y = 90o./ the light travels
along the boundary (do not allow reflects at 90o)
● For angles greater than the critical angle in glass, total internal
reflection occurs
Changes in density or concentration will change the refractive index
Two rays have same frequency/ come from same source/ are
coherent
Path difference (between the two reflected rays)
They superpose when they meet/ constructive and destructive
interference occurs
If they meet in phase, constructive interference/ bright fringe
If they meet in antiphase, destructive interference/ dark fringe
Coherent: Waves of constant phase relationship
Standing wave
No net transfer of energy or pattern of nodes/antinodes or points of
maximum displacement and zero displacement
Superposition along PQ [central maxima in Young’s double slit]
Constructive interference/ reinforcement/ waves of larger amplitude
Crests from S1 and S2 coincide
Amplitude is the sum of individual amplitudes
Explanation of refraction taking place
Change in speed/ density/ wavelength
How do sound waves travel through air?
Prepared by Ridwan Abrar
