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Test 10 (Unit 9)
Geometric Optics. Physics Optics
A. Reading
Read the text. From the list A–E choose the sentence which best fits each
gap in the text. There is one extra sentence which you do not need to use.
The Visible Spectrum and Dispersion
The two most obvious properties of light are readily describable in terms of
the wave theory of light: intensity (or brightness) and colour. The intensity of
light is the energy it caries per unit time, and is related to the square of the
amplitude of the wave, just as for any wave (1;…). Visible light – that to which
our eyes are sensitive – falls in the wavelength range of about 400 nm to
750 nm. This is known as the visible spectrum, and within it lie the different
colors from violet to red. Light with wavelength shorter than 400 nm is called
ultraviolet (UV), and light with wavelength greater than 750 nm is called
infrared (IR) (2;...).
A prism separates white light into a rainbow of colours. This happens
because the index of refraction of a material depends on the wavelength. White
light is a mixture of all visible wavelengths, and when incident on a prism, the
different wavelengths are bent to varying degrees. Because the index of
refraction is greater for the shorter wavelengths, violet light is bent the most and
red the least (3;…).
Rainbows are a spectacular example of dispersion – by drops of water (4;…).
Red and violet rays are bent by spherical water droplets and are reflected off the
back surface. Red is bent the least and so reaches the observer’s eyes from
droplets higher in the sky. Thus, the top of the rainbow is red.
3
Test 10 (Unit 9)
Geometric Optics. Physics Optics
A. Reading
Read the text. From the list A–E choose the sentence which best fits each
gap in the text. There is one extra sentence which you do not need to use.
The Visible Spectrum and Dispersion
The two most obvious properties of light are readily describable in terms of
the wave theory of light: intensity (or brightness) and colour. The intensity of
light is the energy it caries per unit time, and is related to the square of the
amplitude of the wave, just as for any wave (1;…). Visible light – that to which
our eyes are sensitive – falls in the wavelength range of about 400 nm to
750 nm. This is known as the visible spectrum, and within it lie the different
colors from violet to red. Light with wavelength shorter than 400 nm is called
ultraviolet (UV), and light with wavelength greater than 750 nm is called
infrared (IR) (2;...).
A prism separates white light into a rainbow of colours. This happens
because the index of refraction of a material depends on the wavelength. White
light is a mixture of all visible wavelengths, and when incident on a prism, the
different wavelengths are bent to varying degrees. Because the index of
refraction is greater for the shorter wavelengths, violet light is bent the most and
red the least (3;…).
Rainbows are a spectacular example of dispersion – by drops of water (4;…).
Red and violet rays are bent by spherical water droplets and are reflected off the
back surface. Red is bent the least and so reaches the observer’s eyes from
droplets higher in the sky. Thus, the top of the rainbow is red.
3
