Leaving Cert Physics: Waves, Light and Sound

Explain what is meant by the frequency of a wave?

the number of waves passing a fixed point per second.

What is meant by the amplitude of a wave?

corresponds to the height of the wave.

What is a transverse wave?

a wave where the direction of vibration is perpendicular to the direction in which the wave travels.

What is a longitudinal wave?

a wave where the direction of vibration is parallel to the direction in which the wave travels.

What is meant by interference of waves?

when waves from different sources overlap to form a resultant wave of greater or lower amplitude.

What is meant by diffraction?

when a wave spreads around an obstacle or passes through an aperture.

Explain the term constructive interference.

occurs when two waves combine to produce a wave of greater amplitude.

Give two conditions necessary for total destructive interference to occur.

The waves must have the same amplitude and be out of phase by 180⁰ (crests over troughs).

Explain the term coherent sources.

waves which have the same frequency and are at in phase.

What is the Doppler effect?

the apparent change in frequency due to relative motion between source and observer.

What causes the Doppler effect?

Relative motion between source (of waves) and observer

What causes the red shift in the spectrum of a distant star?

Distant stars are moving away from us therefore the frequency decreases and the wavelength increases.

Give two applications of the Doppler effect.

Radar, medical imaging, blood flow measurement (echocardiogram), temperature measurement, (underwater) acoustics, etc.

Explain the term resonance

the transfer of energy so that an object vibrates at its natural frequency.

Explain the term natural frequency.

the frequency at which an object will vibrate if free to do so.

Explain the term fundamental frequency.

The lowest resonant frequency of a vibrating object is called its fundamental frequency.

What are overtones?

multiples of the fundamental frequency

What is meant by the frequency limits of audibility?

These are the lowest and highest frequencies which humans can hear

What is meant by the threshold of hearing?

the smallest sound intensity detectable by the average human ear at a frequency of one thousand Hertz.

Define Sound Intensity

defined as power per unit area.

The sound intensity level at a concert increases from 85 dB to 94 dB when the concert begins.
By what factor has the sound intensity increased?

If sound intensity doubles it means that the intensity level increases by 3 dB, so if intensity has increased by 9 dB then the sound intensity must have increased by a factor of 8.

State the wave property on which the loudness, the pitch, the quality of a musical note depends.

Loudness depends on amplitude
Pitch depends on frequency.

State the wave property on which the quality of a musical note depends.

Quality depends on the number of overtones and their relative strengths.

The clarinet is a wind instrument based on a pipe that is closed at one end.
What type of harmonics is produced by a clarinet?

Odd harmonics

An opera singer, singing a high pitched note, can shatter a glass. Explain why

The glass and the singer's voice have the same frequency so resonance occurs.

Explain the term dispersion.

the splitting up of white light into its constituent colours.

Give an example of the dispersion of light occurring in nature.

Rainbow / oil film colours / soap bubble colours / CD colours

Explain what is meant by a spectrum.

the range of colours present in white light.

Explain the term monochromatic light.

light of one wavelength only.

Explain the term diffraction grating.

consists of a piece of transparent material on which a very large number of opaque (black) parallel lines are engraved.

Name a piece of laboratory equipment used to separate white light into its colours.

(glass) prism or diffraction grating

Can a diffraction grating which diffracts light also diffract X-rays? Justify your answer.

No.
Line spacing must be similar to the wavelength of the radiation (for diffraction to occur) / the spacing between lines in (such) a grating is too large (for diffraction to occur) / for x-ray diffraction, gratings in which lines are separated by infinitesimal distances are required.

Explain how the diffraction grating produces a spectrum.

Different colours have different wavelengths so constructive interference occurs at different positions for each separate wavelength.

Give two differences between what is observed when a narrow beam of light undergoes dispersion as it passes through a prism, and what is observed when a narrow beam of light undergoes dispersion as it passes through a diffraction grating.

Red light deviated least in a prism and deviated the most in a grating
Many spectra observable with a grating, only one with a prism

Explain the term polarisation

the restriction of (vibrating electromagnetic) waves to a single plane

What type of wave motion does light have as indicated by polarisation?

Transverse

Name a type of wave that cannot be polarised.

longitudinal

Why are Polaroid sunglasses more effective than non-Polaroid sunglasses at reducing glare?

remove most of the polarised reflected light (which causes the glare)

What are complementary colours?

pairs of colours consisting of a primary and a secondary colour, such that when combined they give white light.

Only red, green and blue lights are needed to create most lighting effects.
Explain why

All colours can be made by mixing red, green and blue.

List a pair of complementary colours of light.

red and cyan / green and magenta / blue and yellow

How is energy transferred from the sun to the earth?

(by means of) radiation / photons / electromagnetic waves

Light travels as a transverse wave.
Name another type of wave motion and give two differences between these two types of wave motion.
Longitudinal.

Transverse can be polarized - longitudinal cannot.
Transverse waves vibrate perpendicular to the direction in which the wave travels.
Longitudinal waves vibrate parallel to the direction (longitudinal) in which the wave travels.

State the wave property on which the quality of a musical note depends.

the number of overtones and their relative strengths.