P7.2 Flashcards
- What is the speed of waves affected by?
-the medium they are travelling through
- When will wave speed change?
-if a wave moves from one medium into another
- What does a change in the speed of a wave cause? Why?
-a change in wavelength
-the frequency of the waves cannot change (wave speed=frequency x wavelength)
=may cause a change in direction (refraction)
- What would happen if the speed decreases?
-the wavelength would decrease
- What is refraction?
-change in direction of a wave
- How can refraction of light waves be explained?
-a change in their speed when they pass into a different medium= refraction of light waves
- What happens if a light ray hits a boundary (change in medium) straight on?
- slows, same direction
- same frequency, shorter wavelength
- What happens if a light ray hits a boundary (change in medium) at an angle?
-part slows while the other part is still going fast= change direction
- What do convex/converging lenses use refraction for?
-to focus light waves to form an image of an object
- What happens in a converging lens?
-light ray hits surface
=air to glass= slows= bends towards the normal
-light comes out of lens
=glass to air= speeds up= bends away from normal
-curvature of lens= parallel rays bend towards focal point= image is formed
- What is the normal?
-the line at right angles to the boundary at the point where the ray enters or leaves
- What shape is a converging lens?
- it is convex
- fatter towards the middle
- What do converging lenses do to rays of light?
-cause them to converge at a focus
- What is a principal axis?
-a line which passes straight through the lens
- What is the focal point of a lens?
-where rays initially parallel to the principle axis meet
- Where are focal points positioned?
-in front and behind the lens
- What is the focal length?
-the distance between the middle of the lens and its focal point
- What is the relationship between focal length and power?
-shorter focal length= converges more strongly= more powerful
- How do you make a more powerful lens from the same material?
-make it with a more strongly curved surface
- What is the unit of lens power?
-D, dioptres
- What is focal length measured in?
-m, meters
- How does light from astronomical objects reach the earth?
-as effectively parallel sets of rays
- Why does light from astronomical objects reach the earth as effectively parallel sets of rays?
-the astronomical objects are so distant
- What does a simple optical telescope have?
-two converging lenses of different powers, with the more powerful lens as the eyepiece
- How is the eyepiece lens made more powerful?
-it is more curved
- How is a simple optical telescope set up?
-lenses aligned on the principal axis so focal point the same
- What is a telescopes two optical elements?
- objective lens or mirror
- an eyepiece
- What is the purpose of the objective lens or mirror?
-collect light from the object being observed and form an (REAL) image of it
- What is the purpose of the eyepiece?
-produces a magnified image (VIRTUAL) of the image from the objective that we can view
- What do most astronomical telescopes have as their objectives?
-concave mirrors instead of converging lenses
- Why do most astronomical telescopes concave mirrors instead of converging lenses as their objectives?
- smoother so less distorted images
- can reflect more types of em radiation
- mirrors reflect rays of colours the same way
- How do concave mirrors bring a parallel beam of light to a focus?
-parallel rays of light shine on concave mirror, then reflect and converge
- Why are large telescopes needed to collect the weak radiation from faint or very distant sources?
- need a huge objective lens or mirror to collect enough radiation as little radiation reaches us
- diameter of objective lens= aperture
- bigger aperture= more radiation= better image formed
- What is an aperture?
-diameter of an objective lens
- What is diffraction?
-when waves spread out from a narrow gap
- What can be diffracted?
-light
- When is the effect of diffraction on light most noticeable?
-when light travels through a very small gap, comparable to the wavelength of the wave
- Under what circumstances is a wave diffracted most?
-narrower gap or longer wavelength
- What size must the aperture of a telescope be?
-aperture must be larger than wavelength of the radiation detected by the telescope= sharp images
- What happens if the size of the aperture of a telescope is smaller than the wavelength of radiation detected?
-diffraction causes images to blur
- How can a spectrum be produced by refraction in a prism?
- white light disperses as it enters the prism
- different wavelengths of coloured light refract by different amounts
- boundaries aren’t parallel so different wavelengths don’t recombine like they would in a rectangular prism= rainbow not white light
- What is white light?
-a mixture of lots of wavelengths of coloured light
- How can a spectrum be produced?
-by a diffraction grating
- What does a diffraction grating produce?
-a spectrum
- What is a diffraction grating?
-has very narrow slits that diffract light
- How does a diffraction grating make a spectrum?
- light passes through gaps
- different wavelengths of coloured light diffract by different amounts= spectrum of coloured light
- What are spectra from a diffraction grating used for?
-to analyse light coming from stars
