Use a slit light source, a rectangular transparent block on plain paper, pencil, ruler and protractor. Trace the block outline, keep it fixed, and mark widely separated points on incident and emergent light rays. Remove the block and join the entry and exit points to reconstruct its internal light ray. Draw the surface normal vector at entry; measure $i$ outside and $r$ inside from that normal line. Choose a path exiting the opposite parallel face rather than a side edge. Repeat for a range of $i$ with the same block and source colour; record $i,r,\sin i,\sin r$ and repeat-reading spread. Plot $\sin i$ vertically against $\sin r$ horizontally: Snell's law predicts slope $n_{\mathrm{block}}/n_{\mathrm{air}}$, approximately the block's refractive index. The emergent light ray should be parallel to the incident one. Beam width, pencil marks, protractor resolution, block movement and colour spread limit precision; a narrow-colour source reduces spread from optical dispersion. These are measured angular readings, not a direct timing of phase velocity.^[ref]

A normal incidence trial checks the drawn normal and the absence of bending, but the ratio $\sin i/\sin r$ is then $0/0$. Small nonzero angles also make that ratio sensitive to reading errors. Compare several trials through the slope rather than relying on one nearly axial pair. This rectangular-block measurement complements the existing total internal reflection experiment, which probes a different boundary condition.

A submerged object looks shallower because its emerging light rays bend away from the surface normal vector. Their backward extensions give an approximate virtual image nearer the surface. For an object at perpendicular depth $d$ in a medium of refractive index $n_{\mathrm{obj}}$, viewed through a flat boundary from a medium of refractive index $n_{\mathrm{obs}}$, the near-normal paraxial approximation gives $d_{\mathrm{par}}=(n_{\mathrm{obs}}/n_{\mathrm{obj}})d$. Thus the model water depth $12\mathrm{cm}$ appears approximately $9\mathrm{cm}$ from air. This is a bundle-of-light rays statement, not a deflection of the axial light ray.^[ref]

For general transmitted light rays, specify what “depth” means. Put the object at $(0,-d)$ below a horizontal boundary $z=0$. A light ray reaches the boundary at $(x,0)$, with object-side angle ${\theta }_{\mathrm{obj}}$ and observer-side angle ${\theta }_{\mathrm{obs}}$, both measured from the upward normal vector. For $x>0$, its backward extension meets the object's normal line $x=0$ at $z=-d_{\mathrm{axis}}$, whereThis axis-intercept depth is exact for that one transmitted light ray in the stated model. It tends to $d_{\mathrm{par}}$ as the angles tend to zero; at exactly zero the quotient is interpreted as a limit. These elementary tangent, sine and cosine relations show precisely where the small-angle approximation enters.

The original calculations below use $d=12\mathrm{cm}$, $n_{\mathrm{obj}}=4/3$ and $n_{\mathrm{obs}}=1$:

At appreciable obliquity these depths differ, so the full outgoing bundle has no single exact virtual image on the object's normal line. An oblique eye samples a narrow bundle; its local apparent position also depends on that bundle and cannot be inferred by declaring $d_{\mathrm{axis}}$ to be a universal image depth. A plane refracting boundary gives approximate image formation near normal incidence, with departures at larger angles. The construction assumes a flat surface, uniform transparent media with isotropy, and a propagating transmitted light ray; it does not extend to total internal reflection.

At a stationary, uniform planar boundary, incident and transmitted plane waves have the same variation of wave phase along the interface and in time. The figure marks places where equal-wave phase wavefronts meet the boundary. Although their perpendicular spacing differs, their intersections must advance along the boundary together. This is the local mechanism behind Snell's law.^[ref]

For a short derivation, let $x$ run along the interface and $z$ increase into the transmitted medium. Write each wave phase as ${\Phi }_j=k_{j,x}x+k_{j,z}z-{\omega }_{j}t+{\varphi }_j$. The electromagnetic boundary conditions must hold at every $x,t$ on $z=0$; their matching requires ${\omega }_i={\omega }_t$ and $k_{i,x}=k_{t,x}$. Constant offsets ${\varphi }_j$ and wave amplitudes are determined separately. Since $k_{j,x}=k_j\sin {\theta }_j$, this givesFor nonzero tangential wavenumber, the spacing of equal-wave phase intersections along the interface is ${\lambda }_i/\sin {\theta }_i={\lambda }_t/\sin {\theta }_t$. It is not the perpendicular wavelength. At normal incidence, the tangential wavenumber vanishes and the interface is uniform in wave phase.^[ref]

At normal incidence, the incident light ray follows the surface normal vector. Snell's law gives ${\theta }_t=0$ for positive refractive indices, so the transmitted light ray does not bend. Nevertheless, its phase velocity and wavelength change if the refractive index changes. A single axial light ray therefore cannot measure apparent depth: neighboring light rays and their directions supply that information. A reflected component can still be present; no bending of the transmitted light ray does not imply complete transmission.

The angle of refraction is the angle between the transmitted light ray and the surface normal vector, measured on the transmitted side. With positive refractive indices, a propagating transmitted light ray satisfies $n_i\sin {\theta }_i=n_t\sin {\theta }_t$ in Snell's law. The incident and transmitted angles are measured from the same normal line, not from the surface and not from the emergent light ray at a second surface.

The angle of incidence is the angle between the incident light ray and the line of the surface normal vector at the point of incidence. Use the acute angle on the incident side, with $0\le {\theta }_i<90^{\circ }$ for a nongrazing incident light ray. An angle measured from the surface itself is its complement $90^{\circ }-{\theta }_i$. The same convention applies to wavevectors in the transparent media with isotropy considered here.

Consider two dilute populations of spheres with the same buoyant density and radii $b$ and $2b$. Adopt the fixed linear density gradient and constant local centrifugal acceleration of linear-gradient isopycnic focusing, with constant dynamic viscosity and negligible diffusion. Stokes's law makes $V/f$ proportional to $b^2$, so doubling radius multiplies the migration rate parameter by four, although the mass increases eightfold.

Use $u=z/L$, $\tau =t/T$ and $q=({\rho }_b-{\rho }_0)/(GL)=2$. The liquid occupies $0\le u\le 1$, so the matching density position lies beyond its outer boundary. Both populations begin at $u(0)=0.1$; choose ${\lambda }_A=0.15$ and ${\lambda }_B=0.6$. Their band-center trajectories areAt $\tau =0.5$, they lie at approximately $0.2373$ and $0.5924$; at $\tau =1$, at $0.3647$ and $0.9573$. Equal buoyant density has not prevented a useful finite-time separation. Solving $u_B(\tau )=1$ using the natural logarithm gives first boundary arrival at $\tau =\ln (1.9)/0.6\simeq 1.0698$. The corresponding arrival for A is $\ln (1.9)/0.15\simeq 4.2790$. Trajectories describe migration only until contact with the boundary; subsequent collection into a centrifugation pellet is not represented by extending the curve beyond $u=1$.

This is the useful contrast with isopycnic centrifugation: stopping time helps determine the recovered positions in rate-zonal centrifugation, whereas a stationary matching density position sets the limiting band center in isopycnic centrifugation. Real resolution also depends on initial zone width, diffusion and variations in particle properties; distinct calculated centers alone do not prove that fractions will be pure.

Transmittance is the fraction of incident radiant flux transmitted through a specified object or boundary. Specify wavelength, angle of incidence, electromagnetic polarization and which outgoing light is collected.

Radiometric absorptance is the fraction of incident radiant flux converted to heat in the specified receiver: $P_{\mathrm{abs}}=a{P}_{\mathrm{in}}$, with $0\le a\le 1$ for a passive receiver. Specify the entrance boundary, wavelength, incidence angle and receiver temperature. Radiation reflected or transmitted out of that receiver is not absorbed power. A cavity can improve radiometric absorptance by giving light repeated opportunities for wave absorption; a dark coating alone does not establish $a=1$.

For broadband light, spectral radiant flux matters: $P_{\mathrm{abs}}=\int a(\lambda ){\Phi }_{e,\lambda }(\lambda )d\lambda$. A single effective radiometric absorptance therefore depends on the incident optical spectrum as well as on the receiver. If the stated incident boundary lies before a window of known power transmission ${\tau }_w$, the receiver absorbs $a{\tau }_w{P}_{\mathrm{before}}$ when other optical losses are absent. Window loss and receiver radiometric absorptance are separate corrections.

The fraction of incident radiant flux returned by reflection, for specified wavelength, angle of incidence and collection directions.

The 1993 Nobel chemistry announcement credits Kary Mullis with inventing the polymerase chain reaction method and states that the technique was first presented in 1985. The reusable-product principle explains its power: each successfully copied interval can help specify the next generation. The award attribution is a historical claim; it does not imply that every improvement in DNA polymerases, thermal cyclers or PCR controls came from one person.^[ref]

Real-time polymerase chain reaction monitors fluorescence during cycling. With validated specificity, calibration and PCR efficiency, that record supports estimating initial template amount; this quantitative use is called qPCR. Ordinary endpoint PCR amplification examines accumulated product afterwards. qPCR need not start from RNA, and RT-PCR need not monitor fluorescence. Combining both operations gives RT-qPCR.^[ref]

RT-PCR combines reverse transcription with PCR amplification. The RT names the RNA-to-DNA step, not real-time observation. RT-PCR can have an endpoint readout. A matched preparation omitting reverse transcriptase tests whether contaminating DNA could account for amplification; it answers a different question from a no-template reaction.^[ref]

The DNA Learning Center's Detection of an Alu Insertion Polymorphism by PCR by DNA Learning Center, Cold Spring Harbor Laboratory supplies a concrete length-based example. It collects mouthwash biological cells, concentrates them with a microcentrifuge, heats them by Chelex extraction and transfers the DNA-containing supernatant. Its $25\mu \mathrm{L}$ reaction combines $2.5\mu \mathrm{L}$ of that preparation with primer mix and a reagent bead in a thermal cycler. Thirty cycles use $94$, $68$ and $72^{\circ }\mathrm{C}$, each for thirty seconds. An electrophoresis chamber separates products by agarose gel electrophoresis in a 2 percent gel; a UV transilluminator reveals stained DNA.^[ref]

Its PCR primers flank an insertion site at the PV92 locus. The student protocol predicts a 715-base pair product with the Alu element and a 415-base pair product without it. Two bands support a heterozygous genotype; one corresponding band supports a homozygous genotype only if amplification and detection of both alleles were adequate. These are the protocol's expected readouts, not newly collected observations.^[ref]

The instructor manual describes extra bands from nonspecific priming and a small diffuse primer dimer band. A missing target band therefore cannot on its own distinguish failed extraction, inhibited copying, poor binding or lack of the target. Nor does detection of the Alu element at this one genetic locus establish a complete genome, a unique personal identity or a clinical conclusion. The PCR controls help separate these possibilities; DNA sequencing can add sequence evidence to the size comparison.^[ref]

PCR specificity concerns whether the accumulated amplicon comes from the intended interval. Binding temperature, PCR primer nucleotide sequences and their possible alternative binding sites matter. Too permissive binding can support competing products; excessively restrictive binding can prevent the intended product. A band of the expected length is useful evidence, but unrelated nucleotide sequences can have the same length. DNA sequencing tests identity more directly, subject to mixtures and copying errors.^[ref]

For a stipulated population of amplifiable copies, let $N_n$ be the amount after cycle $n$ and define PCR efficiency by the fractional new yieldHere $E_n=1$ means one additional copy per existing copy, hence ideal doubling. If every cycle has the same PCR efficiency $E$, the exponential growth model gives $N_n=N_0(1+E{)}^n$. Starting with $N_0=100$ copies, twenty ideal cycles give $104857600$ copies; the stipulated value $E=0.8$ gives about $12748236$. An efficiency of 80 percent therefore means multiplication by 1.8 each cycle, not retention of 80 percent of the final ideal yield.

In the ideal strand-counting example, all duplex-equivalent material doubles, but exact-length amplicons lag initially. Starting from one longer duplex and retaining its two original strands, cycle $n\ge 1$ contains $2(2^n-n-1)$ exact-length strands out of $2^{n+1}$ total strands. Thus their fraction is $1-(n+1)/2^n$: zero after cycle 1, one quarter after cycle 2 and one half after cycle 3. This counts strands rather than assuming every strand immediately finds an exact-length partner. The generic yield model above ignores this early distinction.

A thermal cycler changes the temperature of reaction tubes through a programmed sequence. A heated lid can reduce evaporation and condensation above the reaction. Instrument timing, sample temperature and nominal block temperature need not be identical.

A PCR thermal cycle separates three jobs. During PCR denaturation, heating separates the paired DNA strands without intentionally cutting their phosphodiester bonds. During PCR annealing, cooling permits base pairing between PCR primers and templates. During PCR extension, the chosen interval supports DNA polymerase synthesis from the paired $3^{\prime }$ ends. Each new nucleotide is supplied as a deoxynucleoside triphosphate; its incorporation lengthens the strand. The DNA polymerase is a catalyst, while these nucleotides are consumed substrates.^[ref]

Taq polymerase withstands repeated high-temperature strand separation better than a heat-sensitive DNA polymerase. Its extension activity still depends on temperature, buffer solution and magnesium ions. Heat resistance does not mean constant activity at every stage: the separation temperature serves the DNA, whereas the extension temperature serves the copying enzyme. As one institutional example, Addgene's Basic PCR Program specifies $94^{\circ }\mathrm{C}$ separation, $55^{\circ }\mathrm{C}$ primer binding and $72^{\circ }\mathrm{C}$ extension. These are example settings, not universal properties of all PCR primers or DNA polymerases.^[ref]

For timing arithmetic alone, those stipulated holds give $20+25+45=90\mathrm{s}$ per PCR thermal cycle, and three cycles give $270\mathrm{s}$. Adding an assumed $90\mathrm{s}$ initial separation and $180\mathrm{s}$ final extension gives $540\mathrm{s}=9\min$ of holds. A real thermal cycler also takes time to move between temperatures. The schedule tells us when conditions favour PCR denaturation, PCR annealing or PCR extension; it does not measure the fraction of DNA copied.

An actual institutional protocol makes the distinction concrete. New England Biolabs specifies mixing templates and PCR primers with its OneTaq master mix in a thin-walled tube; a quick spin collects liquid at the bottom. Its $25\mu \mathrm{L}$ preparation uses $12.5\mu \mathrm{L}$ of the twice-concentrated mix and $0.5\mu \mathrm{L}$ of each $10\mu \mathrm{M}$ PCR primer stock, with water completing the volume. Tubes enter a thermal cycler preheated to $94^{\circ }\mathrm{C}$; mineral oil is specified if there is no heated lid. The program gives thirty cycles of $94^{\circ }\mathrm{C}$ separation, $45$–$68^{\circ }\mathrm{C}$ binding and $68^{\circ }\mathrm{C}$ extension, with extension time scaled to product length. The binding setting must be selected for the PCR primers; the range is not a continuously swept setting.^[ref]

The dilution can be checked independently: each PCR primer has final concentration $10\times 0.5/25=0.2\mu \mathrm{M}$, while the mix is diluted twofold. For an endpoint readout, Addgene specifies examining a product aliquot by gel electrophoresis.^[ref] Pair that size comparison with the existing PCR controls: a known template and a no-template tube test different explanations of success or failure. Those comparisons are an analytical addition, not controls documented in the OneTaq schedule. An expected-sized band supports production of a length-compatible amplicon; it does not establish its nucleotide sequence, ideal PCR efficiency or successful separation during every cycle. Neither protocol supplies a measured temperature or yield curve for the schematic above.