Fluorescence microscopy, discussed in Section 29.8, is an important tool in modern cell biology. A variation on this technique depends on a phenomenon known as two-photon excitation.If two photons are absorbed simultaneously (i.e., withinabout 10-16 s), their energies can add. A molecule that is normallyexcited by a 350 nm photon can be excited by two photonseach having half as much energy. For this process to beuseful, photons must illuminate the sample at the very high rateof at least 1029 photons/m2 # s. This is achieved by focusing alaser beam to a small spot and by concentrating the power ofthe laser into very short 110-13 s2 pulses that are fired 108 timeseach second. Suppose a biologist wants to use two-photon excitationto excite a molecular species that would be excited by500 nm light in normal one-photon fluorescence microscopy.What minimum intensity 1W/m2 2 must the laser beam haveduring each pulse?
Particle Theory of Light
The particle theory of light was the proposal made by Newton in 1704 in his treatise Opticks. This is the most basic light theory, in which light is thought to be made up of microscopic particles known as "corpuscles" and that's why this particle theory of light is also named as Corpuscular theory of light.
Stopping Potential
In an experiment conducted by Heinrich Hertz, an apparatus was made where the incident light was made to fall on the metallic plate, it was discovered that metals emit electrons. The surface electrons are bound to metals with a minimum amount of energy and some of the incident photos enter the surface, they undergo collision with the atoms of the metal, they get absorbed and emit energy to an election, making it photoelectron, where the collision between the photons and electrons ejects the electrons out of the metal and with a negatively charged electron, causes photocurrent and when this current passes it creates an electric field where there is a potential difference at the output due to the anode and cathode of the electrode of the apparatus. This study involves the theory of Quantum physics and electromagnetism involving electromagnetic radiation and electromagnetic wave theory.
Quantization of Charges
An electron is a negatively charged subatomic particle either attached to an an atom or sticks to the nucleus of the atom. Electrons exert the negative charge that tries to balance the positive charge of the nucleus.
Fluorescence microscopy, discussed in Section 29.8, is an important tool in modern cell biology. A variation on this technique depends on a phenomenon known as two-photon excitation.
If two photons are absorbed simultaneously (i.e., within
about 10-16 s), their energies can add. A molecule that is normally
excited by a 350 nm photon can be excited by two photons
each having half as much energy. For this process to be
useful, photons must illuminate the sample at the very high rate
of at least 1029 photons/m2 # s. This is achieved by focusing a
laser beam to a small spot and by concentrating the power of
the laser into very short 110-13 s2 pulses that are fired 108 times
each second. Suppose a biologist wants to use two-photon excitation
to excite a molecular species that would be excited by
500 nm light in normal one-photon fluorescence microscopy.
What minimum intensity 1W/m2 2 must the laser beam have
during each pulse?
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