1
00:00:07,098 --> 00:00:09,959
What if you could only see one color?

2
00:00:09,959 --> 00:00:11,584
Imagine, for instance,

3
00:00:11,584 --> 00:00:13,738
that you could only see things that were red

4
00:00:13,738 --> 00:00:14,869
and that everything else

5
00:00:14,869 --> 00:00:17,106
was completely invisible to you.

6
00:00:17,106 --> 00:00:18,139
As it turns out,

7
00:00:18,139 --> 00:00:20,414
that's how you live your life all the time

8
00:00:20,414 --> 00:00:21,919
because your eyes can only see

9
00:00:21,919 --> 00:00:25,084
a minuscule part of the full spectrum of light.

10
00:00:25,084 --> 00:00:28,183
Different kinds of light are all around you everyday

11
00:00:28,183 --> 00:00:30,101
but are invisible to the human eye,

12
00:00:30,101 --> 00:00:32,940
from the radio waves that carry your favorite songs,

13
00:00:32,940 --> 00:00:35,475
to the x-rays doctors use to see inside of you,

14
00:00:35,475 --> 00:00:37,939
to the microwaves that heat up your food.

15
00:00:38,554 --> 00:00:39,682
In order to understand

16
00:00:39,682 --> 00:00:41,243
how these can all be light,

17
00:00:41,243 --> 00:00:42,656
we'll need to know a thing or two

18
00:00:42,656 --> 00:00:44,284
about what light is.

19
00:00:44,284 --> 00:00:46,519
Light is electromagnetic radiation

20
00:00:46,519 --> 00:00:49,037
that acts like both a wave and a particle.

21
00:00:49,037 --> 00:00:52,028
Light waves are kind of like waves on the ocean.

22
00:00:52,028 --> 00:00:53,991
There are big waves and small waves,

23
00:00:53,991 --> 00:00:55,698
waves that crash on the shore

24
00:00:55,698 --> 00:00:57,235
one right after the other,

25
00:00:57,235 --> 00:01:00,155
and waves that only roll in every so often.

26
00:01:00,155 --> 00:01:02,992
The size of a wave is called its wavelength,

27
00:01:02,992 --> 00:01:04,498
and how often it comes by

28
00:01:04,498 --> 00:01:06,393
is called its frequency.

29
00:01:06,793 --> 00:01:08,748
Imagine being a boat in that ocean,

30
00:01:08,748 --> 00:01:11,231
bobbing up and down as the waves go by.

31
00:01:11,231 --> 00:01:13,667
If the waves that day have long wavelengths,

32
00:01:13,667 --> 00:01:15,789
they'll make you bob only so often,

33
00:01:15,789 --> 00:01:17,696
or at a low frequency.

34
00:01:17,696 --> 00:01:20,058
If the waves, instead, have short wavelengths,

35
00:01:20,058 --> 00:01:21,308
they'll be close together,

36
00:01:21,308 --> 00:01:23,535
and you'll bob up and down much more often,

37
00:01:23,535 --> 00:01:25,286
at a high frequency.

38
00:01:25,286 --> 00:01:27,902
Different kinds of light are all waves,

39
00:01:27,902 --> 00:01:30,406
they just have different wavelengths and frequencies.

40
00:01:31,298 --> 00:01:33,195
If you know the wavelength or frequency

41
00:01:33,195 --> 00:01:34,323
of a wave of light,

42
00:01:34,323 --> 00:01:36,787
you can also figure out its energy.

43
00:01:36,787 --> 00:01:39,002
Long wavelengths have low energies,

44
00:01:39,002 --> 00:01:41,230
while short wavelengths have high energies.

45
00:01:41,230 --> 00:01:42,673
It's easy to remember

46
00:01:42,673 --> 00:01:44,727
if you think about being in that boat.

47
00:01:44,727 --> 00:01:46,428
If you were out sailing on a day

48
00:01:46,428 --> 00:01:47,975
with short, choppy waves,

49
00:01:47,975 --> 00:01:50,398
you'd probably be pretty high energy yourself,

50
00:01:50,398 --> 00:01:53,650
running around to keep things from falling over.

51
00:01:53,650 --> 00:01:54,805
But on a long wavelength sea,

52
00:01:54,805 --> 00:01:57,058
you'd be rolling along, relaxed,

53
00:01:57,058 --> 00:01:58,312
low energy.

54
00:01:59,158 --> 00:02:00,704
The energy of light tells us

55
00:02:00,704 --> 00:02:02,635
how it will interact with matter,

56
00:02:02,635 --> 00:02:04,918
for example, the cells of our eyes.

57
00:02:04,918 --> 00:02:07,630
When we see, it's because the energy of light

58
00:02:07,630 --> 00:02:09,590
stimulates a receptor in our eye

59
00:02:09,590 --> 00:02:11,324
called the retina.

60
00:02:11,324 --> 00:02:13,117
Our retina are only sensitive to light

61
00:02:13,117 --> 00:02:15,319
with a very small range in energy,

62
00:02:15,319 --> 00:02:18,484
and so we call that range of light visible light.

63
00:02:18,484 --> 00:02:20,700
Inside our retina are special receptors

64
00:02:20,700 --> 00:02:22,734
called rods and cones.

65
00:02:22,734 --> 00:02:24,570
The rods measure brightness,

66
00:02:24,570 --> 00:02:26,538
so we know how much light there is.

67
00:02:26,538 --> 00:02:29,552
The cones are in charge of what color of light we see

68
00:02:29,552 --> 00:02:31,221
because different cones are sensitive

69
00:02:31,221 --> 00:02:33,426
to different energies of light.

70
00:02:33,426 --> 00:02:35,421
Some cones are more excited by light

71
00:02:35,421 --> 00:02:37,820
that is long wavelength and low energy,

72
00:02:37,820 --> 00:02:39,654
and other cones are more excited

73
00:02:39,654 --> 00:02:42,537
by short wavelength, high-energy light.

74
00:02:42,537 --> 00:02:43,767
When light hits our eye,

75
00:02:43,767 --> 00:02:46,183
the relative amount of energy each cone measures

76
00:02:46,183 --> 00:02:48,981
signals our brain to perceive colors.

77
00:02:48,981 --> 00:02:50,235
The rainbow we perceive

78
00:02:50,235 --> 00:02:53,621
is actually visible light in order of its energy.

79
00:02:53,621 --> 00:02:54,693
At one side of the rainbow

80
00:02:54,693 --> 00:02:57,397
is low-energy light we see as red,

81
00:02:57,397 --> 00:02:59,734
and at the other side is high-energy light

82
00:02:59,734 --> 00:03:01,631
we see as blue.

83
00:03:01,631 --> 00:03:02,813
If light shines on us

84
00:03:02,813 --> 00:03:04,896
that has an energy our retina can't measure,

85
00:03:04,896 --> 00:03:06,784
we won't be able to see it.

86
00:03:07,600 --> 00:03:09,957
Light that is too short wavelength or high energy

87
00:03:09,957 --> 00:03:11,981
gets absorbed by the eye's surface

88
00:03:11,981 --> 00:03:14,130
before it can even get to the retina,

89
00:03:14,130 --> 00:03:16,940
and light that is too long wavelength

90
00:03:16,940 --> 00:03:17,206
doesn't have enough energy

91
00:03:17,206 --> 00:03:19,576
to stimulate our retina at all.

92
00:03:19,576 --> 00:03:21,520
The only thing that makes one kind of light

93
00:03:21,520 --> 00:03:24,270
different from another is its wavelength.

94
00:03:24,270 --> 00:03:26,215
Radio waves have long wavelengths,

95
00:03:26,215 --> 00:03:28,514
while x-rays have short wavelengths.

96
00:03:28,514 --> 00:03:30,975
And visible light, the kind you can actually see,

97
00:03:30,975 --> 00:03:33,149
is somewhere in between.

98
00:03:33,149 --> 00:03:35,210
Even though our eyes can't detect light

99
00:03:35,210 --> 00:03:37,029
outside of the visible range,

100
00:03:37,029 --> 00:03:38,662
we can build special detectors

101
00:03:38,662 --> 00:03:39,618
that are stimulated

102
00:03:39,618 --> 00:03:41,622
by these other wavelengths of light,

103
00:03:41,622 --> 00:03:43,464
kind of like digital eyes.

104
00:03:43,464 --> 00:03:44,639
With these devices,

105
00:03:44,639 --> 00:03:46,455
we can measure the light that is there,

106
00:03:46,455 --> 00:03:48,804
even though we can't see it ourselves.

107
00:03:50,264 --> 00:03:51,819
So, take a step back and think about

108
00:03:51,819 --> 00:03:53,789
all of this for a moment.

109
00:03:53,789 --> 00:03:54,875
Even though they seem different,

110
00:03:54,875 --> 00:03:57,092
the warmth you feel from a crackling fire

111
00:03:57,092 --> 00:03:58,972
is the same as the sun shining on you

112
00:03:58,972 --> 00:04:00,234
on a beautiful day,

113
00:04:00,234 --> 00:04:01,927
the same as ultraviolet light

114
00:04:01,927 --> 00:04:04,418
you put on sunscreen to protect yourself from,

115
00:04:04,418 --> 00:04:05,997
the same thing as your TV,

116
00:04:05,997 --> 00:04:06,897
your radio,

117
00:04:06,897 --> 00:04:08,280
and your microwave.

118
00:04:09,049 --> 00:04:11,479
Now, those examples are all things here on Earth,

119
00:04:11,479 --> 00:04:13,759
things you experience in your everyday life,

120
00:04:13,759 --> 00:04:16,594
but here's something even more amazing.

121
00:04:16,594 --> 00:04:20,121
Our universe gives off the full spectrum of light, too.

122
00:04:20,782 --> 00:04:22,459
When you think of the night sky,

123
00:04:22,459 --> 00:04:24,057
you probably think of being able

124
00:04:24,057 --> 00:04:26,677
to see the stars shining with your own eyes,

125
00:04:26,677 --> 00:04:28,312
but that's just visible light,

126
00:04:28,312 --> 00:04:30,181
which you now know is only a tiny part

127
00:04:30,181 --> 00:04:31,956
of the full spectrum.

128
00:04:31,956 --> 00:04:33,287
If we had to draw the universe

129
00:04:33,287 --> 00:04:35,600
and could only use visible light,

130
00:04:35,600 --> 00:04:37,428
it would be like having only one crayon --

131
00:04:37,428 --> 00:04:38,702
pretty sad.

132
00:04:39,426 --> 00:04:41,504
To see the universe in its full spectrum,

133
00:04:41,504 --> 00:04:43,329
we need to have the right eyes,

134
00:04:43,329 --> 00:04:45,283
and that means using special telescopes

135
00:04:45,283 --> 00:04:48,158
that can help us see beyond visible light.

136
00:04:48,158 --> 00:04:50,498
You've probably heard of the Hubble Space Telescope

137
00:04:50,498 --> 00:04:51,783
and seen its beautiful pictures

138
00:04:51,783 --> 00:04:54,516
taken in visible and ultraviolet light.

139
00:04:54,516 --> 00:04:55,749
But you might not know

140
00:04:55,749 --> 00:04:58,355
that there are 20 space telescopes in orbit,

141
00:04:58,355 --> 00:05:00,144
missions that can each see part

142
00:05:00,144 --> 00:05:01,780
of the full spectrum of light.

143
00:05:02,379 --> 00:05:04,862
With telescopes acting as our virtual eyes,

144
00:05:04,862 --> 00:05:06,610
both in space and here on Earth,

145
00:05:06,610 --> 00:05:09,260
we can see some amazing things.

146
00:05:09,260 --> 00:05:10,787
And the coolest thing of all,

147
00:05:10,787 --> 00:05:13,201
no matter the wavelength or energy,

148
00:05:13,201 --> 00:05:15,697
the light that we see out in the distant universe

149
00:05:15,697 --> 00:05:17,120
is the same thing as the light

150
00:05:17,120 --> 00:05:19,717
that we can experience and study here on Earth.

151
00:05:19,717 --> 00:05:20,992
So, since we know the physics

152
00:05:20,992 --> 00:05:22,280
of how x-ray,

153
00:05:22,280 --> 00:05:23,410
ultraviolet light,

154
00:05:23,410 --> 00:05:25,204
or microwaves work here,

155
00:05:25,204 --> 00:05:28,202
we can study the light of a distant star or galaxy

156
00:05:28,202 --> 00:05:29,698
and know what kinds of things

157
00:05:29,698 --> 00:05:31,503
are happening there too.

158
00:05:31,503 --> 00:05:33,254
So, as you go about your daily life,

159
00:05:33,254 --> 00:05:35,954
think beyond what your eyes can and can't see.

160
00:05:35,954 --> 00:05:38,371
Knowing just a little bit about the natural world

161
00:05:38,371 --> 00:05:40,347
can help you perceive the full spectrum

162
00:05:40,347 --> 00:05:42,253
around you all the time.