Data Sheet

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96. Conservation of Energy
Build the circuit shown on the left, turn on the switch (62) and you will
see the star LED (70) is on. Now if you touch the reed switch (83) with
the magnet (7), or press the press switch (61), the heart LED (69)
will be on too. In physics, the law of Conservation of Energy states that
the total energy of an isolated system remains constant – it is said to
be conserved over time. This law means that energy can neither be
created nor destroyed; rather, it can only be transformed from one
form to another. In this circuit the energy being lost by the batteries is
being converted mostly to light energy being emitted by the LEDs.
97. Different Color LED Turn On Voltages
Build the circuit shown on the left, turn on the switch (62) and you will see
the star LED (70) is on. Press the press switch (61) and touch the reed
switch (83) with the magnet (7) and the heart LED (69) will be on too.
In project #44 we saw that the frequency of red light is ~451 THz. The table
below shows the frequency and wavelength of all light colors. The table shows
that Blue & Violet are the highest frequency light colors in the 600-790 THz
range. White light is actually a combination of all light colors. There is a one-to-
one correspondence between photons emitted from an LED and electrons that
pass through the LED. Each electron, having a charge q, will fall through the
voltage difference DV (pronounced delta V), using up an amount of energy, E =
q*DV. Each electron emits one photon which has an energy E = hf, with f being
the frequency of the light and h being Planck’s constant equal to 6.626*10
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Conservation of energy lets us say that the energy lost by the electron
is equal to the energy of the emitted photon, so q*DV = hf. So the
Conservation of Energy tells us that for the same light intensity,
higher frequencies require a larger DV, which is why the star LED (70)
producing white light (which contains all colors) requires a higher “turn
on” voltage than the heart LED (69) producing red light.
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2nd level
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Color Wavelength Frequency
Violet ~380-450 nm ~667-790 THz
Blue ~450-500 nm ~600-667 THz
Green ~500-570 nm ~526-600 THz
Yellow ~570-590 nm ~508-526 THz
Orange ~590-625 nm ~480-508 THz
Red ~625-740 nm ~405-480 THz