Which one of the following statements with regard to the ultraviolet light is not correct?
- (a)It is an electromagnetic wave.
- (b)It can travel through vacuum.
- (c)It is a longitudinal wave.
- (d)Its wavelength is shorter/smaller than that of visible light.
Correct — C, It is a longitudinal wave. Ultraviolet light is transverse, not longitudinal, and that is the false statement the stem is hunting for. In an electromagnetic wave the oscillating electric and magnetic fields are at right angles to each other and both are at right angles to the direction the wave travels, which is the definition of a transverse wave. A longitudinal wave oscillates along the direction of travel, and sound in air is the standard example. The other three statements hold. Ultraviolet is part of the electromagnetic spectrum; because the oscillation is of fields rather than of matter it needs no medium and crosses vacuum at 3 × 10⁸ m/s; and it lies just beyond the violet end of visible light, with wavelengths shorter than about 400 nanometres.
- (a)It is an electromagnetic wave. — Correct as printed. Ultraviolet sits on the electromagnetic spectrum between visible light and X-rays.
- (b)It can travel through vacuum. — Correct. Electromagnetic waves are oscillations of electric and magnetic fields and require no material medium, which is why sunlight reaches the Earth across empty space.
- (d)Its wavelength is shorter/smaller than that of visible light. — Correct, and it is what the name says. Ultraviolet means beyond the violet, so its wavelength is shorter than that of visible light and its frequency and photon energy are higher.
The electromagnetic spectrum runs, in order of decreasing wavelength, from radio waves through microwaves, infrared, visible light, ultraviolet and X-rays to gamma rays; frequency and photon energy rise in the opposite direction. All of them travel at the same speed in vacuum and all are transverse, with mutually perpendicular electric and magnetic field components. Ultraviolet occupies roughly 10 to 400 nanometres. It is what causes sunburn and vitamin D synthesis in skin, what the ozone layer absorbs in the stratosphere, and what makes security features on currency notes fluoresce.
Any question that offers a wave type as one of four properties is usually decided on that option alone, because transverse and longitudinal is the sharpest line in the whole topic. The rule to hold is simple: every electromagnetic wave is transverse, and sound is the everyday longitudinal wave. Waves on water and on a stretched string are transverse too, so the mistake is not that transverse waves are rare, only that a mechanical intuition about compressions and rarefactions gets carried across to light where it does not belong. The other honest nuance in this area is that gamma and X-ray wavelengths overlap, and the two are distinguished by origin — nuclear for gamma, electronic for X-rays — rather than by any sharp boundary.
- Electromagnetic waves are transverse, with electric and magnetic fields perpendicular to each other and to the direction of propagation.
- They require no medium and travel at about 3 × 10⁸ m/s in vacuum.
- Ultraviolet lies beyond the violet end of visible light, roughly 10 to 400 nanometres, so its wavelength is shorter and its frequency higher than visible light.
- Spectrum order by decreasing wavelength: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- Sound is the standard longitudinal wave; waves on water and on a stretched string are transverse but mechanical.
- Carrying the compression-and-rarefaction picture of sound across to light.
- Assuming a wave with a very short wavelength must need a medium; no electromagnetic wave does.
- Placing ultraviolet on the long-wavelength side of visible light; infrared is on that side.
As a 'not correct' statements item on a named band, or as an ordering question on the electromagnetic spectrum.
Which of the following are the characteristics of electromagnetic waves? 1. They are elastic waves. 2. They can also move in vacuum. 3. They have electric and magnetic components which are mutually perpendicular. 4. They move with a speed equal to 3 lakh meters per second. Select the correct answer using the code given below:
- (a) 1, 2, 3 and 4
- (b) 1, 2 and 4 only
- (c) 2 and 3 only
- (d) 3 and 4 only
Answer(c) 2 and 3 only
The same properties as a checklist, and it settles the point this question turns on. Electromagnetic waves are not elastic waves of a medium; they are mutually perpendicular field oscillations that cross vacuum, which is why they are transverse and not longitudinal.
CDS_GK_2020_I_Q122020Which one of the following types of radiation has the shortest wavelength?
- (a) Radio waves
- (b) Visible light
- (c) Infrared (IR)
- (d) Ultraviolet (UV)
Answer(d) Ultraviolet (UV)
The same band placed on the spectrum. That earlier paper asks which of four has the shortest wavelength, and the answer rests on precisely the fact stated in option (d) here — ultraviolet lies beyond the violet end of visible light.
- practice — not a real PYQ
Which one of the following is a longitudinal wave?
- (a)Ultraviolet light
- (b)Sound wave in air
- (c)X-rays
- (d)Radio waves
Answer(b) Sound wave in air — the other three are electromagnetic and therefore transverse.
- practice — not a real PYQ
Arrange in increasing order of wavelength: infrared, ultraviolet, visible light.
- (a)Ultraviolet, visible, infrared
- (b)Infrared, visible, ultraviolet
- (c)Visible, ultraviolet, infrared
- (d)Ultraviolet, infrared, visible
Answer(a) Ultraviolet, visible, infrared — ultraviolet has the shortest wavelength of the three and infrared the longest.